a relative had suspected this of his spouse, so what I did was ordered a GPS tracking unit offline that discreetly went into the vehicle. You need to ground it and hook it up to the battery, but once its in there, it works like a charm.
You tell the bitch you are taking the car to get an oil change, washed, waxed, etc, then you install.
Only downside is - the cheaper models are not "live" tracking, you need to extract the device and link via USB in order to download coordinates. but it does work very well!! I believe they do make instant tracking models, but the price is like a 65%+ increase.
good luck!