When I attended the first “Tea Party” on 15 April 09, in Albuquerque, we noticed DHS traveling up and down the street several times during the protest (which by the way the Chief of Police commented later was the most peaceful, well-behaved group of its size he could remember). ANd those were just teh “marked” vans. God only knows how many unmanned vehicles there were. There were also several very conspicuous men in plain dark suits stand around throughout the crowds.
Ive seen that at every tea party event I’ve been to, easy to spot.