@Shadowgreed:
I always assume that he wasn't at the time, even though it was the east blue everybody in the OP world knows who the yonkous are and I believe a yonkou cannot leave his territories without those being attack right away
Agreed. At the very least, it wouldn't seem to go without notice to the Marines/ WG and corresponding areas.
@desa:
They don't know Mihawk or devil fruit. Even Grand line people don't know who Ace is while he was rampaging just three years ago. The level of knowledge is really bellow average there.
They don't know Ace, not for certain; maybe by himself.
But the flipside of Shadowgreed's comment IMHO, would be that in the NW, if no 1(marines, fellow pirates, higher ups in the WH) notices a Yonkou leaving, or the effect of a missing Yonkou and his crew for about a year or so, then Yonkou's don't mean anywhere near as much as they've been built up to mean. But are you saying in the NW, like with Shanks meeting Kaidoh, and Shanks meeting WB, if he goes anywhere really leaving his 'region' , the Marines at least wouldn't take notice and either try to stop him or inform the area it seems he is going to, that he's headed there? That type of news would seemingly spread, like with "the rumors/ Legends" and the competitors in the Colosseum. Eventually every1 would know who he was, especially with bounty pics, and some, if not most, would have treated him differently accordingly, (so it'd be apparent that this was a Yonkou).
Also the point I started at the beginning, maybe they don't recognize a Yonkou alone, but when that Yonkou's traveling around with what's probably the bulk of his crew? Not even 1 person is gonna recognize any of the crew, or even that most of these faces look 'massively familiar'? because "The level of knowledge is really bellow average there"? Don't see that as very likely. If even 1 notices, then they all should be able to figure it out from him telling others.