They'd probably just use whatever bands the local police use, and those communications are encrypted as well. So to an outside observer, they could see that someone is chatting on the police bands, but they wouldn't know if it's the local police or something more interesting.

And these types of events typically establish a no-fly zone large enough that an attacker wouldn't know where they're coming from, even if they knew the exact location of the event (i.e. a campaign rally). It's still possible, but there's a lot that the Secret Service can and does do to mitigate tracking risks.