It's the first notes of the main melody line in the "Mystery" song, from Item Haver: another game that Toby Fox composed music for.

I can already imagine Toby making this

I think we still know only the half of undertale secrets...

wow. I honestly think this is the most layered game of all time

I actually have heard the sound before, but not because of the easteregg but rather by listening to the entirety of the game's audio files in winamp. I always thought it was just some unused sound for some cutscene or interaction, but i guess it never really went "unused".

This is literally the most random shit ever and it's somehow really funny

Ocean: Humans have only discovered the 22% of my secrets so far

Imagine toby putting an ultra hard to get secret and when someone finally finds it it's a Rick Roll.

The even more unbelievable thing is someone thought "oh lemme just type ball on the title screen and see what happens"

Bravery. Justice. Integrity. Kindness. Perseverance. Patience. Using these, you were able to win at "Ball Game."

literally got to know about this just now, anything other at the same level of obscurity?

Hey I've heard this sound on Undertale corruptions

Press shift or x five times on this screen to get some activity measures.

I think it’s a reference to a ball-shaped artifact that you can find in Waterfall, but the dog steals it. The dog being Toby himself, he just hides this ball in this menu

This: probably some joke toby made for the hell of it

Toby is probably laughing his ass off because of how few secrets we've actually found in the game

At this rate even if humanity explores the universe, we'll be closer to seeing everything it has to offer than finding all of Undertale's secrets