Lassiter didn't notice Shawn slipping away. He was used to the man sliding on and off the bed to pee or bring cookies in bed or any number of things, so all he did in response was make a small noise and shift more into himself to make up for the lack of warmth.

However, the sound of Gaster yowling definitely wasn't a usual thing. (She was such a well-behaved kitty.) His eyes snapped open and he sat bolt upright, trying to see through the pitch darkness. Dammit, why didn't Shawn have windows?

And, noticing Shawn wasn't there, he whispered out, "Shawn??" as if the man could just be standing in the bedroom in the dark for no reason whatsoever. Hearing no response, his heart began to pound, various paranoid thoughts filling his mind.

He slipped quietly out of bed and only managed to find and pull on his underwear because he stepped on it, and his gun was on the nightstand, so that was an easy find. He crept out the door, muzzle pointed to the ground.