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?
Until now I haven't seen any news that an exchange has found stolen bitcoins when a user deposited on their exchange addresses, so I don't have any idea how would they find bitfinex stolen bitcoin, maybe after a while they can find but the hacker can exchange and withdraw them in less than a minute!!