How to search a vehicle
Magnetic trackers go where they will stick and not be seen: inside wheel arches, behind bumpers, on the underbody and inside the boot near the spare. A probe lets you sweep those spaces without putting the car on a hoist, which is what makes these different from a general signal detector.
Timing matters. A tracker only transmits when it reports, and some report every few minutes rather than continuously, so a signal sweep that finds nothing on the first pass is not proof. Sweep more than once, and combine signal detection with a physical search of the likely spots.
How to search a vehicle yourself
Work in a methodical order rather than looking where it feels likely, because the point of a hidden tracker is that it does not feel likely. Start outside: run a hand along the inside of both rear wheel arches, then the front two, then under the rear bumper and along the underside of the sills. Magnetic cases are usually stuck to flat steel where a hand reaches easily but an eye does not.
Then the interior. Under both front seats, inside the boot beside the spare wheel, behind the rear seat backs, inside the glovebox, and along the trim at the base of the windscreen. Then the OBD port under the driver's side dashboard, because a plug-in tracker sits there drawing power and takes two seconds to fit.
Last, the engine bay and the wiring. A hardwired unit is spliced into a live feed, so look for tape or a connector that does not match the factory loom, particularly around the fuse box.
Battery, magnetic or hardwired, and why it changes the search
A battery tracker is a sealed box, often magnetic, hidden where someone could place it in seconds and retrieve it later. Those turn up in wheel arches and under bumpers. A hardwired unit is fitted properly and is meant to stay, so it is deeper in the car and takes longer to find. An OBD unit is the fastest to fit and the fastest to find, because there is only one place it can be.
Why timing matters more than the detector
Most trackers do not transmit continuously. They report on a schedule, or only when the vehicle moves, and they are silent in between to save battery. A radio detector can only find one while it is actually sending, so sweeping a parked car that has been still for an hour may return nothing even with a tracker fitted.
Sweep while the vehicle is moving or has just stopped, and sweep more than once. If the detector stays quiet but the concern remains, the physical search above finds what the radio pass cannot.
Related
For finding cameras and listening devices as well, see bug detectors, which cover the full range of transmitting devices.
Common questions
Will it find a tracker that is switched off? No. Nothing detects a transmitter that is not transmitting except a physical search or a non-linear junction detector.
How long does a sweep take? Ten to twenty minutes to cover a car properly.