I have a roof rack on the Subaru. Frequently, I'll put it on the car and go climbing or hiking with my wife then ride home.
Of course the fork mount is locked but I also am able to just barely fit a U-lock through the frame and mounting tray. I've probably left it like this for 6 hours at trailheads and roadside crags. I think it would be pretty simple to get to with a sawzall and drill. Maybe just the sawzall. I certainly hope that someone would notice that.
I'll leave my bike in my truck for days sometimes. It's just a basic camper shell and a locking rear window, not a locking tailgate. It's probably less secure than on the Subaru but isn't as visible.
So yeah, it can be done. I wouldn't do it if it's the sort of rack that can be removed with a knife.