One other thing: Systems typically have talkgroups with a variety of Service Types. For example, the SIRN system in WV is used by EMS, fire, city, county, and state police, the WV Air National Guard, the local schools (security and bus dispatch), and some miscellaneous other things. So with search off, SIRN gets scanned if I have any fire, EMS, law enforcement, or military service type enabled
But with search on, SIRN gets scanned regardless of what service types I have enabled, because the scanner doesn't make any assumptions about what service type unknown talkgroups might eventually be tagged with. I told it to search, and so it searches.
BUT...
regardless of what traffic may be active on SIRN while searching, I only hear traffic that meets one of two criteria:
1: It has a Service Type that I have enabled in the scanner menu.
2: It is unknown--the scanner doesn't know what Service Type the talkgroup belongs to.
First Energy, the local electric company, also has a local P25 system. For obvious reasons, all of the documented talkgroups are tagged with the "Utility" service type. But if I turn Search on for that system, it still gets scanned, when I have "Utility" disabled in the Service Types menu. Why? because the scanner doesn't assume that unknown stuff it might find on that system would necessarily be Service Type "Utility". It could be private security patrolling substations or transmission lines to prevent terrorist attacks. Or any number of other things that wouldn't fall within the "Utility" service type.
So if I have only the Railroad service type enabled, the First Energy system still gets scanned, but I don't hear any traffic from it, because there are no unknown talkgroups, and none of its talkgroups are type Railroad. If I don't want to waste scanner time looking for new stuff on the First Energy system, I have 2 options:
1: Turn off Search for that system.
2: Place an Avoid on the system.
I'll bet the fact that search-enabled trunked systems are being searched, even if they don't contain known talkgroups matching enabled Service Types, is what is confusing you. If you have video of traffic from a known talkgroup being played despite its Service Type being disabled, I'd like to see it.