You don't say what you are scanning when you want it to do this. But, for trunked systems, the channels are never "scanned." The scanner sits on the control channel and compares every active TGID in the data stream with the ones you've programmed into channels to see if the comms should be monitored. So, the idea of "when is the scanner scanning the channels in a particular group in a system" is one that is only valid during conventional scanning.