Some years ago, they were a regular fixture at shows in VA and NC. They were the inspectors(beancounbters) and not special agents. I remember a couple of times when they were handing out donuts.
However, the scope of their knowledge always seemed limited to me. Most questions were answered by "contact the technical division in WVA".
I never saw them walking around looking at tables or otherwise snooping. There were the usual number of conspiricy nuts starting rumors about videotaping or 'spying' on dealers and attendees.
The cops I see in most shows, are there free by badging their way.