Sherlock has been dead a long time and is always around the Sanctuary. He probably sees Sans as a nuisance and more than a tiny bit disconcerting and unsettling, with what he represents to him. But he's got a soft spot for his situation because of being around Helen for 100+ years, so he wouldn't be too rude to him, and would enjoy banter among the lines of attempting to convince him to move on and coming up with increasingly ridiculous excuses.