It's two different detect systems in work, one carrier detect that just checks if there's a signal on the frequency and its level are controlled by the squelch setting, and there's another that goes from what subtone or digital mode you have set.
It will try and decode subtone or digital signals during something like a 250mS time and then give up and go to the next frequency and scan rate will be 4ch/s if the squelch are set to 0, and you only scan systems that use a subtone or use a digital mode.
If it doesn't detect any carrier then it will be no need to check if subtone or digital settings have a match. If the squelch would close during a conversation, it can do that if it receives interference or modulation from the signal are too wide, it will also stop the subtone or digital decode and perhaps continue to scan. To stop that from happening you can set squelch to 0, if you only scan one or two frequencies or sites.
/Ubbe