They don't have Halloween where he's from and Aidan's been too preoccupied to come up with a better costume, so at the end of the day he just picked a hoodie he liked and called it quits. Never expected to actually get actual, movable ears or an undeniable urge to sniff all the interesting smells that's currently flooding his senses.
Like that well-dressed suit guy. Without really thinking about it, Aidan turns on the spot, following along this new, interesting smell.