Not exactly that much of a secret anymore per se but people doesn't know what MC's true job is and his scalings is whack: https://www.mangago.zone/read-manga/the_player_hides_his_past/
Mc gets known to be strong but later, the higher beings like angels and gods that are massively beyond human abilities doesn't know that MC power would dangerously surpass them:
https://www.mangago.zone/read-manga/the_100th_regression_of_the_max_level_player/
Not to the tee with what you're looking for... but OP definitely checks out.
THANK YOU SM ! And it's okay OP is what I'm mainly looking for (ノ≧∇≦)ノ
This manga is really shitty, and stupid, and childish, but when i read this i thought of "unordinary"
Hi please something with a very SECRETLY OP character but PLEAAASE NOT one where he has a super weak ability or is stuck in a super low level or something and is still better with that I'm so tired of that trope