Conflicting messages

There could be an issue with where it's located- could be wireless interference or even the material it's mounted on.  The easiest to try would be to slide the sensor off the base and swap it with another sensor that has been working fine.  If the problem continues at the "new" location then the sensor is the issue.  If the different sensor starts exhibiting the same issue at that location then it's something about where it's located.  You could also try a new battery- on my SS2 system I've seen some random issues like false alarms on motion sensors that I can only attribute to a weak battery, despite not receiving a message on my keypad.