ive heard of several simple ones that can cause this,

as jake say, boot light/ interior light is a favourite when the pin switches fail and short out. anaother is the tracker backup battery that has died, if one of the cells dies it can draw exessive current tryingot charge itself but to no avail and thus flattens the main battery, when ws the last time it had a tracker "audit" when they come and check the movement sensor, transmiter and battery? they recon every 5 years the yuasa dry cell battery needs doing on them.