Oh I think I get it. Please correct me if I'm wrong. If it's the first low ever reported for a given prefix, no scan will be asked. But if a note lower than the current low note is reported after that, it will be considered new and a scan will be asked.
If that's not how it works, it does not explain why for some lows I entered, which are new and which are the first ones entered for a prefix, I was not asked for a scan.