I'm not sure that they wouldn't find a loophole to do this. Keep in mind that in large portions of the country your car and license plate number are already being recorded at traffic lights without the need for a warrant. They could do this on the road leading to the gun show and say that since you're on a public roadway that you have no expectation of privacy and it would pass constitutional muster, as such cases have already done so in regards to other types of surveillance. The grey area, the one where they'd most assuredly need a warrant, would come if they did it in the parking lot of a privately owned convention area. If it were a publicly owned one, county fairgrounds for example, again it would probably pass the constitutional muster under the current situation in DC. The real question that needs to be asked is why a DEA official was proposing such action when firearms do not fall under their jurisdiction?