[quote=@Xhala]
I find myself questioning how ONE character seems to know ALL others . . . Somewhat disturbing, I finde, as it leaves him with an advantage. Where's the mystery? Am I the only one taken aback by this?
[/quote]

As well-liked as Sans is as an Undertale character, he does have some qualities of your average Gary Stu. Some degree of omnipotence and omnipresence, mainly.