I work for an alarm company and we actaully have trackers fitted but rumor has it that the magnets from door contacts can cause the tracker to not function correclty, its all to do with the magnetcic field fooking the gps up apparently. Never tried it though as i dont fancy being the guiena pig and fining otu its a load of bollocks lol. I suppose it may be worth a try, all they can do is pull you in saying your tracker aint working if this works and all you do is take the magnets out before you take it to be looked at.
Apparently my company uses them for engineer deployment (to find the nearest to a job that comes in). But they have now started using them to check peoples overtime claimed etc. which is a coonts trick if you ask me.