We have DIY GPS trackers from Mongoose that track anything unpowered in case of theft. Great for tracking a parked trailer, boat, caravan or a horse float when not in use or not being towed by your vehicle. There are no costly yearly contracts or ongoing fees with the supplier. Just requires a 3G GSM sim card (not included with product), easy set up and a download of the free app will have you tracking in no time. More info and logon here.
Also available for vehicles and personal trackers.