Are you sure, the car unlocks itself? I mean did you really see the
indicators
blink a few times and the doors are getting open by themself. Or do you
just find your car open in the morning?

It sounds stupid, but I had a 306 from 2001. When I parked my car and
pressed the key to close the door, another car, a Opel Corsa opened
and blinked its indicators a few times, when this car was parked close to
my 306. Pressing the same close button again, closed the Opel Corsa.
I asked the dealer about it, he said the RF electronics is for a lot of cars
the same, however it is exceptional, but possible that there are 2 keys with
the same frequency. So mayby somebody is opening your car, when he is
closing or opening his car?? If you don't have this problem by day, when
you park your car where you work and than drive home, you can be sure
that it might be somebody else his key, opening your car accidentily.

i had a similar problem with my dturbo, it started just doing it every now and then, then it wouldn't lock at all, turn out to be the pas. side lock was jamming and had to be replaced.