hmm, second hand, so im guessing youve reset it to your car yeah? or it wouldnt be doing anything at all. i occasionally get a similar problem with my central, in the one door will unlock and not the rest when uisng the passenger side... Have your tried it from the other side door with the key? My problems only intermittent so i havnt really had to look into it.. worse case scenario would be replacing the locks.
Just a side thought, doesnt the locking function work that, when you unlock if the doors arent opened withing (timeout period, 30 secs or so) it will lock itself again, is it possible that your locks are doing the reverse. Id say have a hunt for intrusions but if its unlocking after being locked