For detecting a passive antenna you need a 'bug detector' that includes frequency sweeps. The GPS antenna is still receiving a frequency (I forget the US ones off the top of my head, but, they're picked up by a sweep). First you can do a sweep with the car off and then on (in case the battery has run dead). GPS might be weak inside a building depending on the antenna, so, make sure to sweep outside of the garage if you borrow one.
I have a bug sweeper. Does phones, microphones, cameras, gps (while transmitting as noted before, which they have to in order to do their job, but can't inside, say a four walled concrete building with rebar very well hehe. We played with it at work a bit when we first got it (bugging eachother for fun with harmless stuff that wasn't going anywhere). Now we mostly use it just to sweep walls in engineering, also make sure it's working by testing stuff we know we have bugged ourselves. We send it off to get calibrated every once in a while. There are actually a few different sweepers we have for different purposes, can look at the model numbers if you want on Monday. We don't do it because we're gangsters lol, we do it because its an engineering department.
You'd be surprised how deep you can burry some of the GPSs and get good signal. They'll reflect off pavement. When I check a Pelican case that has some expensive gear that has to go to Europe, it actually has two in it: One US, one EU. I've actually recovered one using this, letting them know my bag went to the wrong city in Spain. They called the airport and found it. Now, the signal I got was brief (probably from the belly of the airplane to some cage inside where it no longer could get signal), but it was enough to track it down. Also, I was actually in another airplane while that signal was transmitted, but they record online so I just logged in from my mobile phone and showed it to the baggage people. Not sure who was more surprised, me that it worked and I got my case (which I really needed for the trip), or them that this gringo had a damn GPS in his bag lol.