My car central locking keeps opening and closing any ideas?

in advance thanks to anyone who reads and responds to this message!

i have an mk4 ford fiesta 99 3dr model. after driving around (as usual) and locking the door i noticed that both doors keeps locking and unlocking (never happened before now), i went to this garage who said it maybe the relay? any ideas if this is the problem? and most importantly is this really expensive as ive taken off the fuse which controls the central locking and didnt wanna bother getting fixed if gonner be alot? even though i would prefer to have the central locking.

thanks again for any responses, much appreciated!