They will likely come back later though

 They will likely come back later though,

because the battery life on active tracker is usually less than 30 days.
Here's where knowing the difference between the two types of tracking devices is important. A GPS Logger (like the iTrail GPS) can only be detected manually - meaning you'll have to physically

search for it. The reason for this is because since they don't actively transmit a signal (they just store data to the device), then a bug detector or bug sweeper won't be able to pick up any kind

of transmission or signal. If you suspect that you might have a GPS logger on your vehicle, the first place to look for it is UNDER THE VEHICLE. This is the most popular place to hide a GPS

tracking device, and is accomplished by placing the device inside a magnetic case, which securely attaches to the metal on the underside of your vehicle. Other places to look are in the glove box,

center console, and the truck (including under the spare tire).
If you suspect that you might have an active GPS Tracker on your car, a manual search is always the first step. Look under the vehicle, in the glove box, and in the trunk. These type of units are

usually bigger than GPS Loggers because they need a bigger battery in order to actively transmit a signal. If you don't have time to do a manual search, you're in luck, because active GPS Trackers

can be easily detected by a GPS Detector.
A GPS Detector is a handheld device that is usually battery powered that allows you to "sweep" your vehicle and will alert you when it detects a

transmitting GPS signal that is sending out your location. There is an adjustable sensitivity knob that allows you to hone in on the exact location of the tracker, so you can remove it from your

vehicle.
A GPS detection device will also detect cell phone signals, because cell phones are commonly used as makeshift GPS tracking devices, since owners of cell phones can often use a "locate" feature of

their phone to find out where it is at.
So if you want to find out if someone has placed a GPS tracking device on your car, first conduct a manual search, and for a quicker and easier search, use a GPS Detector.