"Hm? Oh--don't worry, Sans isn't any form of undead. As I understand it, his race is physically formed out of pure magic; he just has the...hm, unfortunate luck to look similar to human bones."

She also tokes with him on the regular, so that helps her impression of him.

"As for why he was in the ground...he has some form of spatial-warping abilities, and he was high as a kite at the time. I'm sure you can guess from there."