There was a scene shortly before, from Calvin's point of view, where he (it?) is tracking Hugh. Hugh is standing behind the others, who are looking the other way. Hugh looks up into Calvin's POV, the music surges, and then the scene cuts to a shot of David and Miranda, without Hugh in view. Shortly thereafter, he comes into view and then collapses. That's when they finally discover Calvin in Hugh's pants.
The implication being that Calvin slipped into Hugh's suit at that point.
As far as why Hugh didn't mention the monster in his pants, it was likely that he was too stunned/drained/sick to mention it. Or he could have just decided not to (since he felt fatherly towards it).