Zoro unconsciously began flowing CoC Haki out to a consistent and noticeable degree after he first used KOH.
It's not unanimously agreed upon among Zoro fans, but Ashura itself could be intrinsically tied to CoC. Kaido didn't sense Zoro's CoC until after Ashura was used. Therefore, no noticeable CoC was being emitted during their earlier clashes. However, I do think there are other moments in the series when Zoro's latent CoC was felt, but it's infrequent because until KOH it wasn't fully unleashed.
Ashura against Kaku (CoC used)
→ Infrequent emission moments → Zoro uses KOH (CoC fully unleashed)
→ CoC noticeably flows out