While they’re busy loading your bike, hop into their driver’s seat. If you’re lucky, maybe they left the keys in the ignition, free car! If not, either surprise the fuck out of them and ask “Where are we going?”, or call the law, or just beat the shit out of them, your choice.
Paint a stripe or two on your crossbar with distinctive colors. Easy to do, easy to notice & document, hard to duplicate, and hard to remove.