i can't see any of the mentioned or imma too much boomer to notice

O4-at the banner when you choose quest

Note: the devs lied about them all being inside the game.

K2-main page rotating banner

is the T5 in the ultimate level quest title part of the code?

T3 -- On Frozen Chocolate Gacha info description.
This one is not even hidden it's literally in writing next to "Frozen Chocolate!" , so it's different from the rest of the ones listed here.
The picture was posted on the Weekly Gacha thread but don't think anyone noticed it.

w11 on the current mission banner in the events tab beneath the game.