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.
What made you that sure it was yours?
It still had my bike bag on it, which moved it from “pretty sure” to “certain enough to chase that fucker down”. I was told after that the guy was well-known to the cops and had done this before but it’s hard to prove that the guy riding a new bike every day is stealing them. Something you can do is register your bike’s serial number to make it easier to tell if a bike is stolen.
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.