Eddie you are correct, My ex Step-Son had a tag when he came out of prison (Ex because I divorced his Mum and then he was murdered) I saw the whole thing set up, quite fascinating! It is tuned to the tag at both the front door and the back door and furthest point of the house, if he stepped through either door the alarm would be activated, he couldn't even go into the garden, when he answered the door he had to be careful not to step through it.
Out of interest, I examined the tag in great detail to see if it could possibly be removed, I could find no way!, if you even moved the transmitter, the mercury switch inside triggered the alarm, we were told to not even dust it.
He did bang his ankle on the door frame once, we had a call within 5 minutes to see if he was still in the house as it triggered the alarm, 2 hours later we had someone at the house to check he was still in the house.