Alarm goes off after opening door

Since the fob broke off one of my key chains Ive been using just the key. I could use the other set but i think i would just end up breaking it again.

Anyway, the problem is the alarm going off after I unlock the door using my key. It doesn't happen every time but now it does it once a week and it gets super annoying.

Any ideas on how to fix it, what is wrong or how to just deactivate the alarm system all together?