I think you mean vehicles without an OSV sticker?
Here is what the site says:
OSV permits must be permanently affixed to the lower corner of the windshield on the driver’s side, within seven inches (7”) of the edge of the windshield. The expiration month sticker must be affixed before adhering to windshield.
Assateague Island National Seashore - Over Sand Vehicle (OSV) Use (U.S. National Park Service)
Maybe you looked on the bumper????
You can call the Rangers at 410-641-1441 if you don't see one.