It might have been somebody who purchased a car on a tourist delivery plan. Most of the major Euro carmakers who sell cars in the States have schemes whereby you pick up your US-spec car somewhere in Europe, drive it on your vacation and have it shipped back to the States when you leave. IIRC, you recieve European plates for use on your trip.
When you return, the car is waiting for you on the dock and you simply claim it and drive off. Since the car is US-spec, it can be registered without any hassle as if you bought it from a dealer in your town. I don’t remember if you have to go to the DMV yourself once you get home or if US registration is done for you as part of the process. Either way, it may have been somebody who got the car in Europe and hasn’t gotten his or her US tags on the car yet.