From the Product Manual:
All Door/Window Sensors and Motion Sensors report in to the Console every 90 minutes.
If the Console does not receive a signal from the sensor within four hours, it reports a
problem with that sensor by slowly flashing the appropriate zone indicator. If you try to
arm the system with a zone indicator flashing, the Console sounds a repetitive trouble
alarm. Either press DISARM, correct the problem and press ARM again, or press
BYPASS while the trouble alarm is sounding, then press ARM. The zone(s) with a
problem will not be armed and the zone indicator(s) will flash rapidly. All other zones will
arm.
If you bypass an open window and arm the system, and later close the window, that zone
will now be armed.
The report is the "Heartbeat" or "Signal"