I'm speaking for what i know using a BC796D.
When in search mode, it will pick up ALL talkgroups, whether you programmed or alpha tagged them. If you have not programmed or tagged anything in a system, it is the ONLY way to listen to that system.
I leave mine in search mode most often. When listening to a particular service, I lock out other banks, switch to scan, lock out unwanted talkgroups by group, then I can listen only to those talkgroups which I am interested at the moment.
Steve