Sans noticed. Undyne noticed. Hell, even Toriel and Asgore had noticed. Alphys knew from the beginning but left it to sit. The general consensus was:

Papyrus was acting weird.

Not regular, Papyrus weird. Like - VERY weird. Sans would see him typing, and instead of the boisterous "NYEH HEH HEH", Sans would hear soft giggles, coupled with an orange face.

It all came to a head when Papyrus was late for a gathering. Late?? Papyrus? Now everyone was concerned. Sans had been told that Papyrus had 'errands to run', so its not like he was in immediate danger. But, still. This was very unlike him. So when he entered the room, skull orange and a giddy smile adorning him, he was the centre of attention.

"HELLO, ALL. I SIMPLY HAD SOME...DUTIES TO ATTEND TO, NOT TO WORRY."