What alerts you to you roommates presence?

He comes up behind you

You didn't notice until it was too late. Kevin came up behind you and grabbed your hips, pressing himself up against your ass.

Shocked, you jumped into the closed refrigerator.

"Kevin! What are you doing here?" you asked nervously, turning your head as far as you could to face him, feeling his erection between your butt cheeks

"Work got out early. Figured you might be playing around. Turns out I was right, but I never expected you to be so hot," Kevin responded, his fingers massaging your flesh over your shirt.

"It was never any of your business," you snap, stuggling to get free, but failing.

Your body starts feeling aroused, something not altogether new to you, but you were always thinking about girls then, not cocks, like the one slowly rubbing up against you.