Did the house predate the bathroom?
If so, the bathroom space may have originally been part of the kitchen, and there may have been a wood or coal cookstove in the kitchen.
Re-Stumper: Best job title ever.
The house was built in 1934 as a cabin, and is in a seaside holiday community (i.e., not for year-round use), so that is what I suspect. There’s also a pipe in the back yard that is certainly an old well. The house has had at least three additions. For example, when we re-did the middle bedroom we saw that there was once a window behind where the stove is in the kitchen. We’re pretty sure the bathroom, laundry room, and back bedroom are additions, so I suspect the mystery chimney was outside of the house. The newest addition is the front bedroom, and based on the roof structure the ‘dining room’/living room was added.
We had a wall switch that was a mystery for a short while. The fireplace mantel (on the other side of the room) had three electrical outlets on top. The center outlet was always ON and intended for a plug-in clock. the outlets at either end were controlled by the wall switch. They were for electric candlesticks.
There is an old story about people who moved into a house and found a plywood board with two legs screwed onto one end. There was an piece of paper taped to the bottom: “This is for when you paint the stairwell. This makes a level platform on the stairs. And go across the street to borrow the ladder from the Smiths.”
In the future people are going to find lightning arrestors for land-line telephones and wonder what they were.
Or in the present. (I had to look them up.)
Mine isn’t as interesting. I was doing some work at my mother’s house when I found a flask shaped liquor bottle stuck above the utility sink in the partially finished laundry room. I thought it was from one of my older siblings being naughty as a teenager. Then I noticed the tax stamp was dated 1959. One of the construction workers had a liquid lunch when he was building the house.
And what did you study at Notre Dame? 
If we’re talking electrical things, my house is a big electrical WTF.
Whoever finished the basement was a good enough carpenter, but a poor electrician. Half the outlets were installed with the polarity reversed, which means they were just wired randomly.
The rest of the house is even worse. Some circuits are copper, some are aluminum, and some switch in the middle. I had to pull every single receptacle, switch, and fixture to figure out what needed remediation.
The biggest WTF to me was one of the bedrooms didn’t have the ground hooked up. I finally found the problem in the other bedroom. The ground wire to the bad bedroom didn’t quite reach the box where the two rooms’ wiring were connected, so it was just dangling unused. I added a pigtail to make it longer, hooked it up, and then the whole room was grounded.
No mystery switches, but there is a mystery outlet in the pantry that doesn’t have power. I can’t pull the faceplate to see if it’s actually wired without removing a built in shelf.
UCLA, actually. Sink-pooping must be a nationwide craze.
My parents had a mystery switch in their garage. It was next to a door that entered into an office and since my parents never used that door they just put shelves in front of it. The switch didn’t turn anything on or off and my dad always assumed it was disconnected.
One day we were doing some minor repair in the house and were using a voltage sniffer to determine what rooms were on which circuits. Out of curiosity I went and tested the mystery switch in the garage and surprise! It was still hot. Obviously that just made the mystery deeper.
We still don’t know what it went to.
For myself, when my wife and I bought our house we saw it 3 times before we closed and got the keys: the first time we looked at it, again a week later after our offer had been accepted, and finally when we had the inspection done. The house had been vacated a few months before it went on the market.
The second time we toured the house there was a package on the doorstep, addressed to house but with a name that was not the previous owners. The return address was the same name and address, so the post office couldn’t return it. It was probably 2’ square on a side and heavy: 30lbs at least.
When the inspection was done 10 days later, the package was still there.
After we had closed and we went to what was then our house for the first time, the package had been moved into the garage by the previous owners. I messaged my realtor and told him that we were getting rid of anything that had been left behind (and there was easily a big pickup load of junk left behind) and this was the last chance for the previous owners to get anything they might want. He messaged back and said they had already done so and everything left was ours to do with as we pleased.
So everything went to the dump. Including the mystery package.
Fast forward two months. I’m telling my friend about this and she freaks out. She tells me this is what drug dealers do to move their product around: the buyer finds a house that’s empty, has the dealer ship it to said house, and the buyer watches for the package to arrive. She theorized that the house went on the market after the buyer provided the seller with the address but since it was sold within hours the buyer got cold feet and never picked it up. Which is weird as the package was sitting on the porch for at least 10 days before the previous owner moved it into the garage.
According to my friend who I’m confident knows a lot about such things, if it was pot it was worth about $150K. Anything harder would be worth many times that. But whatever was in that box, it’s now buried in the landfill somewhere.
Your kidding! That was my first draft, but I actually dislike ND more.
Fight On! 
Crawlspace of our old house held a few interesting finds. Cork stoppered Rx bottles, intact rodent skeleton, buffalo nickels and an old pipe wrapped in longjohns that had the name Jim sewn in the collar.
We unwrapped the longjohns and underneath was an old newspaper from November 19, 1915. A headline read “Ominous silence believed to portend doom of Serb Army”. A precursor of WW1
It was June of 1992, headlines of that year were portending a Bosnia Croat war.
This reminds me - the crawlspace of our previous house had a cat skeleton in it. There were also “albino” cave crickets - albino in quotes because they weren’t truly albinos, just mainly white with a few brown spots on them.
We have a mystery vent on one of our outer walls that doesn’t do anything.
Around 1960 my brother found a can full of silver certificates (a few hundred dollars worth, IIRC). in an out building near the house. I think it turned out the previous owner’s late wife had put them there.
Renovating the middle bedroom, we had to pull up the carpet. It was nailed to the floor. Double rows of nails (about 2 inches apart), about a foot apart from the next double row, and the nails within the rows a couple of inches or a few inches apart. After ripping up the carpet, I spent a lot of time on the floor pulling each and every rusty nail.
I did a remodel for a buddy, and under the linoleum was 1/4" ply that had been stapled down 1" OC, no kidding. Either the MT Dew or the meth was good for that install. Lots of hands and knees trying to get them all out!
I find it amazing that you didn’t open that box before throwing it out. I can’t…even conceive of doing such a thing, throwing away a box without knowing what’s in it. How in the world were you not interested enough to at least find out what was in the box!?
Exactly! I definitely would’ve opened it! it could’ve been the 150k payoff for the box of drugs that was already delivered.
Honestly I just wanted it gone. The previous owner had left a lot of junk behind and we had to spend a long weekend cleaning it out. Not just random boxes but some appliances and furniture too. The box was just another piece of junk left behind. It wasn’t until I mentioned this to my friend – and she told me about the whole drug shipping thing – that I realized it could’ve been valuable.
And if it was drugs I’m kind of glad I remain ignorant.