Bogus open entry reports

New user  here: While away from home this weekend I received a call from SS telling me that there was both a motion sensor alert and an entry door trigger. It being 1:45 AM, I instructed them to send the police (who found nothing to report). This afternoon at 1:35 PM SS called again to say there was a motion (no entry this time) alarm. I opted to disregard it. When I came home today I checked all four doors and found them to be secured properly. When I opened each one the base station chime operated as designed, so apparently the fault isn't in the sensors.

I assume that both of the motion sensor alerts could have been the doing of our cat, but why did I receive that 1:45 AM call reporting an entry breach when no door was opened? As an aside we had an ADT system here for six years and never had a false alarm. BTW, the SS motion sensors are placed exactly where the ADT units had been located.
Any input would be appreciated.
Rick