Yep, could well be the solenoid as there is a microswitch inside which triggers the CL system for the rest of the doors. There's also one on the back of the manual lock barrel.
Chris, do you normally deadlock the doors or just a single press? Does this make any difference to whether it unlocks itself? Is the driver's door deadlocking properly?
Another question - if the doors unlock themselves, is the alarm still armed? i.e. does it go off if you open the door or is it disarmed - because only reception of a paired keyfob signal will disarm the alarm.
I tend to always dead lock it with two presses. I'm fairly sure it been deadlocked when I've found it unlocked, ESP in a public car park.
I've never noticed any oddities with locking this particular car. All the buttons appear to drop at the same time. No bouncing back up or double attempts to lock. Only problem I've had is being locked IN, where I've locked the car from inside when driving through a dodgy part of Slough, then once on the m4 I've tried to unlock to stop the right rear door pin rattling, and its refused. The drivers door knob was firmly down and refused to be pulled back up again. I had to open the window and put the key in the door which was extremely stiff.
Re the Alarm, I'm not sure. It's never gone off if I open the door, but I tend to lock and relock the car on return if I find it unlocked to a) make sure its still working to see did it respond to me locking it and b) check my sanity.
But I'll try to open the door next time, and see if the alarm goes off.