It is scanning the frequencies you programmed for that particular system looking for a control channel.
If that suddenly happens it means that the control channel changed to one that is not programmed and the scanner can't find it.
OK thanks. That's what I thought. I wasn't sure if it only looked at the frequencies programmed for the system or looked at some other method and determined it should be the one. Im my case the cc changed but is too weak a signal to be detected.