Multiple cordless and cell phones and a wireless router sometimes enabled.
The security remote for any X10 security system, or dialer system, transmit on 310MHz. Your cordless phones, cell phone, and wireless router all use much higher frequencies, so they should not be a problem. But there is always Murphy's Law. The wireless router is the only listed device that transmits all the time, so unplug it and try to arm the SC1200.
Other users have had similar complaints and seemed to have solved by replacing the suspect sensor. Because of the number of similar stories, I wonder if X10 does not have some "bad build" units in the field(?).