I used to see this many years ago, but not in the past 5 or so.
I passed a FHP trooper who had pulled someone over. My Redline alerted to a very weak Ka signal when I was passing him, perhaps 50 feet away, then it went away. The trooper was out of his car. I recall years ago that some radar units would emit very weak signals when in 'standby' mode.
The only other explanation is that he was using a hand-held radar gun that was still transmitting, but was laying on the seat next to him, so only a tiny bit of signal was escaping his vehicle.