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?