Twice the FBI has broadcast the presence of their surveillance vans by being really stupid. They actually name a wireless network, broadcast from inside the van, "FBI Van 1" or similar. And they keep getting caught.
Recently when they busted the terrorist/idiot in Round Rock:
"[Neighbors] said it appeared that as many as 10 people were living in the house, and lately there had been some signs of suspicious activity: When residents looked for available Wi-Fi networks, networks with names like "FBI Van 1" would pop up."
And a few months back, the suspected bomber posted about it on Facebook before getting busted.
If they ever track real terrorists they're risking the lives of those agents. And being really, really stupid.