If a mixer receives coins that are from a known theft, as far as I know they remain neutral to the whole thing and limit themselves to do their job (mix the coins). There are certain sites like Helix that as far as I know are fully automatized, run on a onion tor website and deliver clean coins, after that I would say it's impossible to find, so he would proceed to sell in exchanges. Of course if he isn't a total idiot, he will not dump and will sell slowly so he doesn't get caught.
So in this situation... how can they find them? I wish they find them and people can recover funds but what if he runs coins through mixers?
The mixers does not have the potential to mix all the bitcoins that were hacked so probably the hacker will start mixing the hacked bitcoin by starting with a small amounts so they can't be traced, I don't think that the bitcoin mixers will blacklist these addresses like the exchanges because they have profit from this.