Searching to see if there are more terrorists engaged in a coordinated attack? Seems like a reasonable and responsible thing to do.
I, and at least 12,000 others was listening on police band (over the internet from 3 states away), and was able to watch on TV while listening to the scanner traffic, both apparently delayed by nearly the same amount.
On the scanner, several police units reported being "slow rolled by a car full of long beards". There were several different incidents of this with words to the same effect, "watching us closely", "seem way too interested", etc.
They even passed these car descriptions and plate numbers to others to be followed, but that was before the two terrorists were engaged in a rolling fire fight, and killed, and everybody seemed to focus on that.
So its clear the police believed at the time there were others (long beards) surveilling them. And it wouldn't be out of the ordinary for that to happen. Even the Paris attacker had returned to the scene to watch.
Ok, so the radio plane was flying around 2:30 PM (14:30) local time, very soon after the event. An attack is still in progress, for all they know. Can't think of a single reason to complain about this, other than that you know they practice and train for it SOMEWHERE.