If it is not wet all the time it can’t be something with water pressure all the time. Does the shower back up to this wall? Is their a vent pipe from your drains going up one of these walls? What is on the other side of the walls where the wet spot is??
Here’s what’s happening. I think…
There is likely plastic between your drywall and the insulation in the attic. The plastic will act like a funnel and guide any water under the insulation to the lowest spot. The light fixture. The leak can be anywhere in the rafter space running the length of your attic. There is a leak somewhere up there.
And here I was thinking it was about off-season water levels. 
Carry on.
Reading Kayaker’s last post I have learned some things.
Since the towel on top of the plastic was dry, & the carpet underneath the plastic was wet, the water is not coming from the light fixture.
I believe, like Kayaker, that the water is coming from the attic down the wall. Now all he has to do is to find the trail of water & backtrack it to the source, the leak in the roof. Easy peasy! 
He may have to wait for more rain & climb up into the attic with a flashlight during a rain storm to find it.
I am thinking there is a leak in this system that is running to a low point and pooling.
How about burning down the house? Would that take care of it?
I’d check your roof for broken/cracked shingles. That’s what we finally figured out was causing the leak around one of my new kitchen windows. We were almost at the point of taking the window out and redoing it. Now I have a nice shiny new expensive roof instead.
One of the most devilish leaks I ever faced was intermittent wet spot in our basement in a little 1940s bungalow in Seattle. The wet was not associated with rain, but it was beneath the bathroom. I caulked the tub, then fittings, then looked everywhere else until finally I discovered that the overflow drain to the tub had never been connected to any plumbing. Any water that got in there ran down the inside of the wall and exited somewhere on the stairs below. That explained the intermittent nature of the leak. We never ran tub full to the overflow, but depending on who was showering and how long, some water inevitably got out the overflow and inside the wall. I had to take a wall out of the stairwell to get to it. Discovered an empty beer bottle in the wall. After the leak was repaired, I rebuilt the wall and left an official Dangerous Radiation Area sign in there from Idaho Nuclear Engineering Laboratories. Did anyone ever find the sign? Years later my son and I went back to Seattle and drove by our old house – workers were just putting the finishing touches on a new house where the old one had been. So maybe.
Must be a thing. The first night in our new home, my wife took a bath. I was sitting on the sofa downstairs when it started raining inside. She liked to keep a slow trickle of water running, and once it hit the overflow, it worked its way downstairs. Had the plumber out, he found a never connected overflow (along with apparently a very confusing set of pipes that were working so we just left it. Not enough of a plumber to understand/remember what he was describing 20+ years ago).
This is what I’m thinking, kayaker. What’s on the other side of the wall from the wet spot? If it’s a bathroom, I’d start looking there. It could be a small leak from the toilet coupling, or the wax seal, or maybe the shower/bathtub. A small leak could get under the wall and start soaking the hall carpet.
Reading back through your posts I’m starting to suspect the bathroom more than anything. I see there is one close to the hall but no water lines near the hall, but leaks can be very insidious and travel unseen along walls until they either find an outlet or something nice and absorbent, like a carpet.
That would be my bet. We had water come out the bedroom ceiling fixture every great now and then and since the roof was old and all I assumed that was the issue even though the attic seemed pretty darn dry. One roof later we still got the same thing now and then. Turned out to be bad flashing/sealing around the one chimney and the water would sometimes find its way down along the bricks, across a beam between the attic and second floor along the electrical lines :eek: and out the fixture. Two tubes of caulk, a trip to the roof, and no problem since. Water does some strange things.
Help troubleshoot my mystery wet spot
Do you have any history of sleep-walking, -peeing, or -happy endings?
On the first night in our humble little home after our humble little honeymoon I awoke to find the bed was WET!
As I was planning on returning her home to her parents she was awake thinking it was ME!
Turns out the roof leaked!!
We’ve both had lots of good laughs and this was only 45 years ago. :<)
No, That bathroom is the guest bathroom, last used over xmas.
Went up into the attic and may have found the problem. I cannot say with certainty, because there is no decking in the direction of the problem. I rearranged the stuff store in the attic, an ID’d the general area that is involved.
The roof at that spot is complicated. The sunroom addition abuts there. If the soffitt/fascia there is to blame, water would leak right into the spot where the the spot is located. Looking around in the attic there is no other explanation.
Now another issue. The roof is only 4 or 5 years old. So, I contacted the roofer and we talked. He suggested alternate explanations (all of which have been discussed in this thread) and I explained why I do not think they are viable explanations. Eventually he agreed that the roof/soffitt/fascia needs to be checked. I told him I do not expect a free inspection, but just want to get to the bottom of things.
We are a good customer ( have referred a few others). He is going to stop out later in the week. I suspect maybe leaves in the gutter/ice dam/etc.
I was sort of hoping the explanation would be more ----- Satanic? 
------ but my bet is that you’re pretty much right.
If there’s a leak in the roof, the water could be finding its way along the ceiling and finding an exit opening - through that light fixture. At our last house, there was an issue with a slow drip in the bathtub in the kids’ bathroom (I don’t know whether it was the drain or a feeder pipe, but we had to cut a hole in the wall of the closet in the adjacent room to get to it). It manifested by occasionally dripping through one of the recessed lights in the living room.
Are there any pipes in the ceiling? You sure about that? Go up in the attic and check out what’s near that spot. Check for any evidence of moisture near that spot even if there are no pipes.
Oooh - and utterly unrelated to the OP’s situation but we had a baffling wet spot in our basement, years back (same townhouse as the dripping light fixture). It was on a side wall, about 2 feet back from the front wall. We thought it was because of a downspout, so we had that corrected to drain further away from the house.
Then it recurred.
And it was happening on days when there had been no rain for weeks.
There were no pipes anywhere near it, though we checked out anything nearby. Naturally this occurred when we had put the house on the market, and it nearly cost us the sale. The buyers were literally about to walk away… and the neighbor knocked. Turns out, the air conditioner’s inner portion, in the furnace room, had a pipe to handle the condensate, which was simply laid across the floor and pointed at a floor drain. Theirs had somehow gotten knocked out of place, gone unnoticed for several years, and was puddling against the wall between our two units. PHEW!!!
The other dampness issue we had at the same time, which might be more relevant: a spot in the finished basement ceiling. That ultimately turned out to be our own fault: we had put mesh screens over several vent openings because birds had taken to nesting in the kitchen exhaust fan pipe. Unfortunately, it didn’t occur to us that doing this to the dryer vent was a bad idea. That pipe - which ran some 12 feet through the ceiling to the outdoor opening - had gotten stuffed with years of dryer lint. When we figured that out, we wound up with several bags full of very old, soggy lint, and a LOT of drywall that had to be patched. EWWW.
So anyway, are there any vent pipes that go through that ceiling or the floor? These might either directly allow water in from the outside, or could be trapping damp dryer lint.
My bathtub had a crack in it that was discovered when my daughter ran water for a bath, went downstairs to grab a magazine and noticed water leaking out of the kitchen ceiling light fixture.
The roofer finally came. He insisted that the problem was likely something other than the roof. He wanted to look up above the basement ceiling tiles (which I’d already done). So he looked and agreed with me that all was cool there. Then he suggested a pipe in the attic (which I’d already ruled out), so he went up, looked, and agreed that wasn’t the problem.
Eventually he went up on the roof. He came off the roof and told us there was a problem. He was kind of pissed at whichever of his employees did that part of the roof. He said there were difficult angles from the sun-room’s roof attaching there, and he said he did not recall working on that part.
He spent a few hours on the roof and when he finished he said all was good now. I got out my checkbook, but he said I owed nothing. I offered to pay for supplies, but he pointed out (correctly) that I’d probably spent more just on aggravation over dealing with the problem.
Looks like all is well now.
Sounds like this is a roofer you should hang on to and recommend to others and write nicely about him on line. I hope this is really the fix, anyhow!