For a moment, Sans's expression is exactly as it always is when he greets the human at the first bridge. Just neutral, waiting to see what happens, how it's going to go this time.

And this time the kid recognizes him.

That technically never happens, and normally that would be cause for alarm, but here? Here that's just good news. His grin widens and he offers his hand.

"so you do know me. good to see you, frisk."

Or it could be bad, but that's what all this song and dance is for.

"bet you've got a lot of questions."