Alarm going off with key in pocket

[FredOxford]

C3 Aircross auto petrol flair no options
At least 3 times the alarm has gone off when I do the following.
Key in pocket
Car is unlocked
As I open the door to enter the car, the lock locks. As the mirrors close the alarm go's off.
At least one of the occasions it's my mum opening the passenger door. Perhaps always having the car locked first would avoid the problem but that is not ideal.

[Jay R]

Are you close to the car at the time? Perhaps you can leave the car unlocked but the alarm still goes off if someone tries to open it when the keyholder is not around?

[FredOxford]

Yesterday I returned to the car after paying for petrol at petrol station. (Wrongly did not lock it) As I opened the door it not surprisingly triggered the locking process then the alarm went off.