Yeah I think I'll go with both the tracker and hidden switch or plug like suggested. At least hunting out of coverage it will still slow them down.
I've decided to give this a crack: https://teltonika-gps.com/products/trackers/fmc234
8 days on it's own backup battery and three outputs for the (air) horn/alarm/fuel/start etc bit of reading there to figure it out though.
Cheers for everyone's thoughts.
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