Any wireless device can miss a message or drop a message due to signal issues, collisions, etc.  An alarm message is only sent a few times so if you were out of range that alarm message could be missed.  Vulture Systems added in the possibility of setting your devices to 2 way mode just for this reason.  POLL Mode allows you to periodically poll the unit in case any alarms were missed.  If a device is in alarm and if you POLL it you will get that missed alarm.  POLL Mode also gives you the piece of mind so that you know you are still in range of the Sensor.  When in doubt or bored just do a POLL for that extra piece of mind!