Are they away with their national teams? In July 2012 it was the Euros weren't it? maybe they're there as your preseason is fairly early
Germany national team doesn't call up real players on FM unless you allow it.
Maybe they missed the flight....
Did they also play in a reserve game on the same day? It may say in the player detail screen why. It also may just player is unavailable for the next game, but worth a shot.
Training camps consist of players leaving to have special training together. You get to choose who go there and in the message I think it's told to you, it can be a different country (in a Valencia game I started they went to Rennes, I think, in France). So they're unavailable because they're away.
By the message when they returned, their main use is to build team cohesion, which would be gelling, right?