I’m late into the discussion of the timing of the missing persons report but it’s also possible it began with Libby’s grandfather’s attempts to determine where the girls were through the tracking of her cellphone. In one if the interviews he mentioned he personally knew one of the police officers and had contacted him, perhaps for advice or to determine if LE was aware of other possible information such as accidents involving unidentified teens etc, so it would make sense if that officer strongly encouraged him to file a missing persons report at that time.
So I think the report wasn’t filed because the families were deeply panicked or desperately frantic, more so because of the benefit of LEs assistance in their search to help find the girls. It clearly is a close-knit community because in other places I’d be really surprised if police would even bat an eye at a report of two teens gone missing for only a couple of hours. JMO
Good points and also I'd add, MP said he was buddies with some firefighters so that's another level of first responder who was "on it," right from the beginning, a connection that most people, or at least most people in bigger communities, would not have had.