If you are into the apple ecosystem I can highly recommend AirTags. They work amazingly well, dont require a data plan and the battery lasts around 2 years. I have one in my bike.
AirTags are entirely passive in their tracking and are essentiell bluetooth LE beacons. If they get in the range of an Apple device (eg iPhone) with active Bluetooth, the Apple device reports it's own position together with the ID of the AirTag to Apple.
Since they announce their presence actively to function, they are stupid easy to find by the thieves by using a detector app and they only work in the first place if there are enough phones around to keep the position semi updated