If you only want to listen to programmed trunked radio ID's then put the scanner in ID scan mode. If you want to hear all traffic on a trunked radio system then put it in search mode. In ID Search mode to save a TGID to the radio press Eyes key when it is active. You can quickly escape back to scan mode by pressing L/O key.
If you are interested in detecting nearby transmissions you can put the radio in CC Pri mode where it checks for nearby transmissions every two seconds, if you choose CC DND then if you are scanning a system and there is no transmission received the radio will check for nearby CC transmissions.