For me, up to that point when he entered that room, Brett believed that he was hunting for a small creature. However, upon further reflection, I agree with you that he acted stupidly when, after finding the shedded skin, instead of reporting his discovery to the crew, Brett decided to proceed further into the room, and placed himself in greater danger. Another moment that can be argued for a case of stupidity is when the captain went alone into the vents without a partner. So, in conclusion, I retract what I said earlier about the first movie.
Well to be fair, even with the shedded skin Brett had no reason to believe the creature would had grown THAT much from before. And animals like snakes shed their skin for lots of other, more mundane reasons than just their growing size.
Plus, he probably just didn't want the others calling him a pussy for running away.