Preventing my trunk-mounted bike rack from being stolen
The metal brackets that clip on the edges of the trunk lid cannot be removed when the trunk is closed and locked, so no-one is going to casually unclip the rack and walk off with it.

I suppose someone could lever the brackets out, but that would damage the trunk lid and surrounding bodywork, so you would have a bigger problem and an insurance claim.

Also the straps could be cut, but what is the point of stealing a ruined rack?