Have a Back2You passive GPS tracker, very good and perfectly fit the needs.
The GPS has a mobile number, you ring it and hang up. Then you receive a text with GPS coordinates and and a GOOGLE Maps link. Plus you can set a "movment" alert, if the tracker leaves a 100m circle you get a text immediately.
The advantahe that it is a passive one so cannot be found with GPS locators or scanners, it emits a signal only when you call it, sends you a text and goes back into passive mode.