I bet:
When Zoro and Mihawk finally fight there will be a short flashback or inner dialogue. Mihawk will remember the first time Zoro whipped out his 3000 worlds attack and, unconsciously, used CoC. The scene with Mihawks sweat drop.
Impressed by someone as weak as Zoro having such CoC he decided to spare him so he could fight prime Zoro in the future. Mihawk is not somebody who kills potential, on the opposite he had been depressed for years as no one had been able to match him like Shanks, you know the guy who was the first to figure out Luffys potential.
Goes without saying that unless a swordsman has CoC of a certain level the other guy could never beat Mihawk so Zoro must have used it. Logically Mihawk must have sensed Zoros unconscious use of CoC all the way back then.
And of course, the other guy who also used it unconsciously extremely early was Luffy. Hawkeye Mihawk, perhaps #1-3 CoO user in the series, obviously noticed. Same as Fujitora, another #1-3 CoO user, did in Dressrosa regarding Luffy and Zoro.