There's enough bandwidth to go around nowadays that alarms can send regular keepalives (which doesn't mean all of them do).

If the keepalives stop coming without a proper disarm signal, a fault is raised.

Some old alarms had a weaker version of this, where they would dial the security company whenever the door was opened, and then again when the alarm was disarmed. If the second call didn't come in time, the company would instantly know that something was up.

This protected against thieves that would enter the house and smash the alarm before it had time to activate.

These days, alarms use quantum entanglement. Beat that :)