Even on high end racks, it is possible given some time.

My rack locks to my car and my bike rails lock to the rack.

After that, the QRs that hold the bike lock so that it cannot be easily removed.

I'll leave the bike on the car while eating somewhere while traveling, but I would not leave bikes unattended in a remote parking lot or overnight.

The locks can eventually be defeated and somebody could just strip expensive components off the bike.