Gaius was always... Perceptive, if that's the proper term. He noticed things. Like when a student suddenly appeared on his roster.
Some of his students noticed too, a questioning look sent to the seat where the man with the hoodie in a sea of slightly formal clothing had placed himself. He waved with a tired, but cheeky, lopsided grin.
Class continued normally.
Afterward it was like there wasn't a time without him.
And then he was gone.
Like his very appearance never happened.
It wasn't until Arthur, the son of his oldest friend, called him to ask if he had been to class, did he consider the fact that something wrong.