Given what is currently happening in the US, I cannot help but ask myself: Does the police still respond when emergency calls come in reporting that several armed and masked men are dragging people into a van and kidnapping them?
That's a genuine question, and I don't know the answer. But hey, I don't know the answer to the question of how all this can be possible in a constitutional state either.
In this situation, it can't be just one person. Several people need to call 911 with plate numbers and general descriptions.
"A bunch of people in masks waiting outside the court house. I think they have guns!"
If ICE has done their due diligence, they would have notified law enforcement that they are there in plain clothes. But methinks that these cosplayers wouldn't think to do that.
If you really want to be an inconvenience, DON'T give them the plate numbers. Those will almost certainly come back to some entity or person that will clue the dispatchers in on them being government owned.