Sans, for the most part, is doing his best to pretend he isn't present at all.

It's worse than he had even anticipated, hearing these two go at it. The kid is messed up, and bad, and the sound of Greg's ambient misery is the next best thing to torture. He doesn't really belong here, right now. This has nothing to do with him, and there's nothing he can really do about it.

Except for the part where, at some point, he's going to be the one that has to separate them. Right now, it's hard to imagine Greg cooperating with that, but ever moment that ticks by is another moment that their absence could be discovered, or that some sort of alarm he had no way of preparing for could go off.

The anxiety is nauseating. So, as best he can, he focuses on thinking about nothing.