S.G.
you can get gps tracking on her cell phone as well as her car. you can explain to her that she should not have a problem with this if there is no reason not to trust her and as long as she lives there, this is whats going to happen.
my cousin does this with his daughters.
also he has an alarm in the house that activates the front door space camera when the girls arrive home from school. they do not know about it. it shows if they have people come over or not. so far they have been really good about telling the truth. but if they ever did lie, that camera would tell them and there would be no way to get out of it.
a lot of security companies install these now as a package and it isnt that much more a month. we had some installed and its something like $48/mo.
cell phone companies will do that for you if youre on a family plan or paying her bill.
personally, if my kids grow up and i have a trust issue, yet im paying their bills - i would have zero problem monitoring them in this way. i do not consider it a violation of privacy. i consider it a protection of safety.