Earlier models of GPS devices & low-cost trackers have had fairly large magnetic boxes and external antennas making them hard to be kept hidden. With the latest designs, except for plug & play OBD GPS devices, the possibilities of finding an installed GPS device is very less unless you intend to find it at any cost with or without the help of an automotive technician.

GPS devices (with in-built antennas) can (almost) be placed in covert ways, so it’s not so easily found out. However, an out of place wire or a wrongly placed antenna could be a giveaway.