I saw in a Dolphins article that Manning was addressing the team about the importance of studying and not taking anything for granted.
I think Manning is coming to Dolphins camp because he and Gase are good friends and I'm sure Gase loves for him to talk to the QB's, especially Tannehill. If you are trying to read something else into Manning visiting Dolphin training camp, it just isn't there. Manning loved playing football and I am sure he still enjoys helping out any team which welcomes him to speak to their players. I'm sure he has probably visited other players to discuss the coming season and not all those players are on the Dolphins.
Peyton has nothing better to do, but let's not be naive, I'm sure before he left he spent some time with RT and the qb's to give some insight. But yes he was in NO last week. Not a big deal