Proctas Defender

[Dum-Dum]

Also take a look at TrackR

https://www.thetrackr.com/

Hide one somewhere in it. You just wouldn't believe the lengths people go to in order to steal Defenders. You can put all the mentioned security on them and they still get stolen, it really is crazy.

Those only work if a user goes within 100ft so if it gets taken out of the city you might not find it.
If you don't pay for a proper GPS tracker you can hard wire in an old mobile phone and some companies will let you track it for a few quid via triangulation off the nearest masts, this covers a much greater part of the country, it's not necessarily as accurate but it would certainly get you close enough for the trackers JJ suggested to work.

I saw it done in an old issue of PPC.