In the sparse time it too for him to be woken up by an elf, surreptitiously dress, and make his way through the directions McGonagall gave him where she waited outside the Slytherin Portrait Hole, Draco went from groggy to very much over alert. McGonagall had only said he was meeting someone, and as he followed the directions and came out into what he could also assume was the Shrieking Shack - wasn't this meant to be haunted? - he had narrowed the list of possibilities down to... well, no one.
The look of surprise when he saw Snape there was quickly schooled away, though, and he immediately put his occlumency training into practice, closing that part of him that could register surprise at all.
"Oh," he said after a moment. "I would have suspected she would have killed you, rather than letting you meet with students at one in the morning."