Tampered sensor triggered alarm in the middle of night

The alarm went off at 4:58 am, when looking at the app, and after searching my house for someone, it showed that the kitchen door sensor was tampered. In fact the battery cover was not fully closed (which I've noticed happens as a result of constantly opening and closing that door), but how did this go undetected by the base station (there's nothing in the event history after the last time the door was closed) and why would this trigger the alarm hours later after Home was activated? This is a second generation setup.