2010 Focus Zetec alarm going off when open door from inside.

[ChampionRabbit]

Hello!

Probably being an idiot.

Wife has new/old car (2010 Focus 1.6 Zetec).

If you drive the car, then get out and then a passenger opens a non-driver door, it sets off the alarm. Why is the alarm even armed at this point? Seems very strange; must be a fault?

Any ideas would be ace!