It's kind of a puzzle. [Not usually Sans's thing, but he can make an exception for this.] They definitely mean something, like writing, but there's no answer key, so whatever they say's still a mystery. It's where we showed up, so it could be related.

[Sans can hear the whispering again, but only barely. The others at the ruins don't notice when it picks up around Sans for thinking about the symbols, so hopefully this means Papyrus can't hear it either--Sans isn't sure how he'd explain that part.]