LoL's hidden stat

facepalm here you go: http://leagueoflegends.wikia.com/wiki/Ashe

Frost Shot FROST SHOT: Ashe's basic attacks and abilities apply Frost Shot Debuff Frost to affected enemies, Slow icon slowing them by 5 - 25 (based on level)% for 2 seconds and causing subsequent basic attacks against them to deal 10% + (critical chance × (1 + bonus critical damage))% AD bonus physical damage while they remain Slow icon slowed.

CRITICAL SLOW: Ashe's Critical strike icon critical strikes deal no additional damage, but instead double Frost Shot Debuff Frost's Slow icon slow strength to 10 - 50 (based on level)%, decaying over the duration to its normal strength.

Ashe doesn't crit but does bonus damage on targets effected with her frost passive. It's not that it's hidden so much as I don't think it's explained very well lol.

You mean an Ashe with 100 AD attacks a target with mini-crits from her passive, boosting her damage?

There's modifiers in champ abilities everywhere. With Ashe, it just happens to be in her passive.

Ever notice how a 100 AD Ashe will deal 120 damage on autos?

Yes. That's her passive at work.

There is a hidden multiplier in most, if not all champions that no one seems to notice.

There isn't supposed to be. If it did exist, it either be a mechanic explained on the champion's kit (like Ashe), or it would be a bug. If you think there some modifier does exist, we're going to need to have examples of what you mean, and we'll need to see some proof.