Here is the answer I got from Fraport this morning about the mystery shuttle bus. The seem to have a great customer service team.

"The shuttle bus operates between 6:00 AM and 2:00 PM without a fixed schedule and is available exclusively for transferring passengers with EU air security status, such as those from the US or for passengers from non-Schengen EU countries, for example, Ireland. Authorities determine which countries of origin are granted an EU air security status.

However, it also depends on where you are traveling onwards to.

The bus departs as soon as its full and transports passengers from departure area A/Z to B and vice versa.No other connections are possible with this shuttle. It is quite possible that you may still have to pass a security screening if you do not belong to the previously mentioned group of persons."

The 2pm end time is a bit puzzling, but I suppose it's aimed at the TATL traffic.