If she heard cackling in the bushes? Like seriously?
Well she'd know what she did in the first scene, but this forbidden forest one...why would someone go in there? Okay, well she had in her first year, but that was when she didn't know any better! She grew up practically in a forest at home.
"What if you disillusioned yourself? And then...kicked the bush hard before running?" RUN!. RAN Ran ran...all the way back to the common room, because obviously a professor would somehow pop up out of nowhere and give you detention. It was inevitable! It happened allll the times.
Sorta.
ooc: didn't see Lockhartian's post before editing
gah sorry for sorta stealing that answer. haha.