in that case, it is the "interior sensor" trigger the alarm, as the ultrasonic detected the wave of the electronic device.
or.....could be a fly inside your car cabin kept flying and flying after you armed the alarm ?
as for the "friend finder", yes.....people have been doing this long time ago. used to have a case in JB, a stolen Camry was found using this service. refer to the below link, we discussed before.......