I fitted a tracker device to my bike today. It is this one https://www.amazon.co.uk/dp/B01KJS55JS/ref=pe_385721_37986871_TE_item and it fits in the housing of the controller unit powered by the bike 36v battery. Using a payg sim all I have to do is send it a text and it sends back a google map link of the bike's location. It does seem to work. There are additional functions too which I haven't explored yet. Probably not foolproof but for less than £30 it at least gives a sporting chance of tracing the bike if it goes awol. It surprises me that there is so little discussion of these devices on the forum because an electric bike seems like an ideal application for them. Perhaps you all have them but sensibly want to keep it a secret!