It took Sans "Has Never Told Anyone a Single Thing About Himself Even Once" The Skeleton a full unbroken 30 seconds to come to the conclusion "Oh. Papyrus probably mentioned it off-hand."
(He did in fact play it off like he was sleeping with his eyes open)
I think that Undyne puts a lot of work into having good rapport with her subordinates, even if she doesn't particularly like them (if they REALLY bother her she just rants about it to Alphys later). That's her TEAM, y'know? Especially when they're close to someone she's close to. (can't stand Mettaton, but that's Alphys' friend so he's allowed in her house)
I think she would've put in a little extra to trying to get along with Sans because of her friendship with Papyrus. Only to be met with The most infuriating brick wall of a guy. This isn't high on her priority list, she probably doesn't think of Sans much beyond "Paps' brother" and "why the fuck is he assigned to so many stations". But at the same time, I think she did perk up at the mention of the two of them having something in common.
She hoped that she could get some energy out of the guy by trading scar stories (just like in anime!!) or talking about how cool eye patches are or commiserating over the trials of poor depth perception.
Unfortunately for her, Sans is the most guarded motherfucker on the planet who lives exclusively by the Mulaney Code: "I'll keep all my emotions right here, and then one day, I'll Die." The man is allergic to the concept of confiding things in others.
(For clarity of headcanon I like to think that Undyne is entirely missing her left eye and Sans is blind in his right eye. The pupil still manifests, but it's not actually doing anything. He doesn't, like, actively hide it, but like I said he is incredibly stubborn about not letting anyone know anything about him ever so he'll Not Mention Things for as long as possible until he literally has no choice.)
Putting these two in the same room is unstoppable force vs immovable object. "Feels things so hard she literally melts herself" and "Feelings can't get me if I keep dodging"
They should fight about it with weapons. And also be best friends
