2002 s60 2.4t

The problem:

Door locks work as expected. Except when the rear driver's side door is opened and shut. Then, pressing lock on the keyfob locks all but that door!

If I unlock all 4 doors with the keyfob, then hit lock again, all 4 doors lock.

The only other symptom I can see is when I close the right driver's door & this happens, there is a little sliding sound, almost as if they system is trying to lock itself (?)

Just to clarify, the lock handle (you know, the thing you push on when you're manually locking the door?) does not move down at all when this happens. It seems truly unlocked. The door does open, and the alarm sounds.

(I say this because i just read a thread where the locking handle DOES go down yet the door does not lock; My problem is not this. I did not want to confuse anyone)

Thanks for the help!