
I've been thinking:
If Paget comes back, too, I bet we find out Rossi figured out that Prentiss is still alive. He saw how J.J. stopped Reid from going in to see her. He might have also looked up to see Hotch walk out of the room and how he & J.J. had that little talk. As unemotional as Hotch is, he's still pretty readable to those who know what to look for.
We all figured out he knows. Hotch figured it out with one clue. Rossi has had two.
Rossi, just seems to be in the right places and available for Hotch to talk things through, should Hotch need to. Giving Hotch a chance to talk during his own assessment. Then being there when Strauss was on the phone ragging on Hotch.
I think there is a possibility, that if Paget comes back, that Rossi will just say, "Oh, I knew." And if Hotch says, "Why didn't you ever say anything?" He'd say, "For the same reason
you didn't. But, I was always available should you have needed to talk to someone about it."