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?
If they get run through mixers, it does help to change up some of the ways you can trace the coins and it makes it considerably more difficult to determine which address holds the funds, but you could probably still find them if you really wanted to track the person down. You just have to spend a lot of time checking each address and seeing if any sync up to moves from the original wallet.