picasso central locking keeps unlocking itself?

Citroen Picasso 1.8 Year 2002. I have had a problem with the central locking, It will either take a few attemps to lock with the fob or the key or locks first time but after an undetermined time the car unlocks itself.can take a few hours or just a few minutes.When the doors are locked while the car is traveling it generally unlocks after a few minutes. Have had the central locking system checked by a citroen dealer today who could find nothing wrong but they have lexia tested actuators & parameters,removed & checked all door wiring & connectors,found no brokenwires lubed all connectors of door & boot all ok, But have got home and the car has unlocked itself as before. Has anyone got any ideas as i am totally fed up with it.