OMG can't believe I didn't think to implement this sooner! So, enemies hear your footsteps! It means 1) you gotta be sneakier, and 2) when they're chasing you, they can easily follow your footsteps! Suddenly, they're better at tracking you down, instead of standing there confused from afar when you slip around a couple corners
So depending on if their alert level is low, medium or high, they'll be able to hear further.
PS I'm almost done with the second level! And here's a random screenshot of enemies finding me hiding under a desk: