Portia was entirely unsuspecting as she stepped out into the cold January afternoon but she thought the chill of the air was good. She had been spending entirely too much time studying, particularly since she didn't have any major exams hanging over her head like the 5th and 7th years did. She found stepping outside for a bit was definitely good for clearing her head and it didn't hurt that not many people chose to go outside on the cold winter days. Thus, she was entirely oblivious to the fact that she was walking directly into a trap.
Though, if she had been paying more attention, the great big wall of snow probably would have been a pretty good tip off to the fact that something wasn't exactly right.