Does this alarm get transmitted on a public safety freq?
If so, sounds like a Varda alarm. Varda made (maybe still does, I'll Google it) a portable alarm unit that included an HT, audio playback device and sensors for the alarm.
Trip the alarm, and the HT would key up and play back whatever was recorded onto it's tape playback device.
I had Milwaukee PD install one of these in my workplace a long time ago. Sure enough, the caught the guy that had been repeatedly breaking into the place. The idea is that the alarm went right to the cops with absolutely no delay - no alarm company, not getting stuck in dispatch, nothing.
The drawback for the listener is that you'll have no idea what alarm is for. The cops will, of course. That Varda alarm went someplace else the next day, and they never changed the audio tape. All it ever said was "Varda alarm, Varda alarm, Varda alarm."