Read this post, then do the first step, and if you think you are up to it do the second step also.
Step 1
Take and put the remotes somewhere in the house at least 50 feet from the car. They won’t be able to talk to the car at this point and will be out of the equation. Then verify that the trunk is closed. Then start the car with the valet key.
Does the trunk pop open at this point? You may have to try this a few times to be sure.
Step 2
If it does pop open, then get into the passenger seat and crank your head around and look at the front side of the rear view mirror. You will see a green rectangular electrical connector. In the center of the bottom, you will see a tab that can be pressed. With the ignition key off, press the tab and pull the connector straight out. Be careful, you can rip the mirror off the car if you are not careful. You have now disabled the UEM, your alarm, central locks and a few other things will no longer work.
Now try starting the car again, you can use the any of the keys, because the remote receiver has been disconnected.
Does the trunk pop NOW? If yes, the problem is with the REM, wiring, trunk switch, or relay.
When you are done with this experiment here is what you will need to do to reset everything to full operation and not have any residual fault codes.
Turn ignition key off.
Wait at least 2 minutes.
Plug the electrical plug back into the mirror
Open trunk
Flip up mat at the rear of the trunk.
If the battery has a full cover and a brace over it, remove the 2 13mm nuts on the front of the brace, and loosen the one 13 mm bolt at the rear. Remove the brace and plastic cover.
If there is no black plastic cover continue to the next step
Using a 10 mm wrench remove the NEGATIVE battery terminal (Usually the one closest to the center of the car)
Leave the battery disconnected for 10-15 minutes.
Reconnect and reassemble.
make damn sure you pull the covers back over the battery and/or the positive battery cable back in place. If you leave them off, a fire can result from a short circuit
Restart car.
Check for fault codes by doing the following:
With the key on or the engine running press and hold in the message button on the end of the turn signal stalk.
While holding it in quickly press the rear fog light button twice (obviously this will take two hands)
Let go of the turn signal stalk.
The message center on the left side of the dash will display Checking DTCs
Then it will display either NO DTC in vehicle, or DTC in Vehicle
each time you press and release the message button on the end of the TS stalk a different system will display. You will see one of three messages
XXX Checking =OK
XXX No DTC =OK
XXX DTC set =Problem in that system.
XXX= three letter acronym for various electronic systems in the car.
Report back
All right, line 'em up. Clearly the engineers that drifted off into designing these fobs branched from the same tribe that brings us most modern software. Ready, aim, fire!
Well when you compare it to what it takes to program the remote on postcards’s car, you will probably want to line up the the guys that designed the programing on your car and kiss them.
For a newer car
[ol]
[li]Hook car up to 7,000+ diagnostic machine[/li][li]Order software from central database for remotes ()[/li][li]Download software into car to open the car’s programing function[/li][li]Enter the 8 digit alphanumeric code (case sensitive)[/li][li]Now enter the 16 digit alphanumeric code (again case sensitive) (thse codes come with the remote. If you loose them, or buy a bare remote of eBay you are screwed.)[/li][li]done.[/li][/ol]
Still want to shoot the guys that designed your system? 
Bloody Millennium Falcon only needed a screwdriver and a hammer to fix anything. Cars are getting to big for their boots 
Rick, Mrs. P here.
Thank you for your sage advice and detailed instructions. Unfortunately, it will still be a few days before Mr. P can try them, as, not only is the key fob out of range, but the Volvo is out of range by about 300 miles. I do want to share my current situation with you, however. The car trunk unlatches when the ignition is turned on and off every time all the time. The good news about where I am in Vermont is that there is a low crime rate and I have nothing of interest in the trunk. Battery theft is unlikely. Due to the fact that it is windy and about 10 degrees (very sunny, thank you), it is a bit painful to get into the car and then have to get out of the car and slam the trunk before driving. Thus I’ve devised a satisfactory temporary solution:
Late winter causes frost heaves, and the start of the spring thaw brings rivulets cascading down the dirt road, forming humps, bumps and serious gouges. If I start the car and leave the trunk unlatched, I have only to head downhill about 300 yards before I can aim for any of the newly-created obstacles in the road. When I hit the dip, the car drops, the trunk lid goes up, the car comes up from the bump, and the trunk lid slams tight.
I confess this is a low tech solution, but it seems to work with great consistency.
Much thanks.
No problem, I will stay tuned for further developments. Even though I don’t work for Volvo full time any longer, I want your car to be right and you to be happy with it.
I love your low tech solution.
I know that this is a very old post and i am brand new to this forum; however, i am experiencing the exact same problems as described here. Any further help or answers that weren’t posted would be so greatly appreciated.
Well have you
A) Read the entire thread?
And
B) Done all the steps listed?
If so where are you at? What were the results of the tests?
I can tell you there is a 99.99% chance you won’t be able to fix it at home, and a better than even chance many shops won’t be equipped to handle it.
Where are you located?
Rick = Awesome
TRAVERSE CITY MICHIGAN. THANKS SO MUCH FOR YOUR REPLY
Yes I have read the full post. Yes all three keys unlock the trunk in position 3. Nothing else is going on with the car. It does not happen when the ecm is disconnected.
The messages that you asked for are as follows
DTSC IN VEHICLE
BCM CHECKING
SRS READY
TCM CHECKING
ECM CHECKING
DEM DTC SET
AUM NOT CHECKED
CEM DTC SET
DIM READY
DDM DTC SET
CCM CHECKING
PDM DTC SET
REM DTC SET
SWM DTC SET
UEM DTC SET
And since I started this thread, and realize that I never followed up with Rick, I should check in and report on the situation.
When the weather got warmer, the problem went away. Never happened again. I thought it may have been a cold-weather thing, but it never recurred; not in the winter of 2008-09, nor during 2009-10. In the spring of 2010, she traded the car in for a BMW X3.
Apropos that, my rear screen would unlatch on every start up last (very cold) winter but has never done it since.
When you say it does not happen when the ECM is disconnected do you mean when the UEM (rear view mirror) is disconnected? The ECM is the engine control module which is under the hood, and requires a special tool to disconnect.
I’m guessing you mean the UEM. Further when you say all three keys, I am going to guess that you have two keys with remotes and 1 valet key (no remote) So if starting the car with the UEM connected with any of the three keys causes the trunk to unlatch we can rule out the remotes, as the valet does not have a remote function.
If the problem goes away with the UEM disconnected, then we can localize the problem to the UEM or its programming.
At this point we have three possibilities. In no particular order they are:
[ul]
[li]A wiring problem at the UEM, this would either be a chaffed wire or more likely a bad ground wire to the UEM.[/li][li]The programming to the UEM has gotten corrupted (happens) and can be cured with reloading the software[/li][li]The UEM is hosed, and will require replacement and new software.[/li][/ul]
If I had to bet on these in order or probability, I would guess software first, with wire and bad UEM tied for 3rd.
Except for checking the wiring, you are way out of most shops depth here. For software or UEM replacement you will need a shop that has the VIDA tool to be able to communicate with the car and download software. This is going to be either a dealer, or a Volvo specialty shop.
If I can answer any further questions, feel free to ask.
postcards thanks for the update, I did think your wife’s work around of driving on the frost heaves was very creative.
Any suggestions for a volvo mechanic in Northern Michigan.
replace the rear trunk switch assembly 
Hey, 100.00 bucks an hour !! Problem is in the rear latch. Communication issue causing trunk release on startup. I make 22.00 an hour 
There is obviously a zombie in the trunk trying to get out. Sheesh.