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!!
they can tag the known addresses with dirty bitcoin on them in their code (it is a simple process) and so if they receive any funds from those knows addresses the exchange blocks the account immediately.
but the question here is about using a mixer, and i think if it is used in smaller amounts and used correctly then it can be possible to become untraceable.
Then let's imagine this scenario: If a hacker transfer the funds from A to B and then send from B to exchange address(C) then the exchange will not be able detect that the funds are from those stolen?
I'm not sure how they can detect but if it is only for direct payments from dirty addresses marked as to known dirty address then this is not secure to find the hackers.