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?
You can't run 120,000 BTC through mixers. There just isn't enough liquidity in the world across all mixers to even begin to launder that amount of bitcoin. Sure, if he wanted to mix 50 or 100 BTC here and there -- and he would probably want to run them through multiple mixers -- he could maybe do that. But he would need to be
very, very careful. Several blockchain analysis firms are watching those coins, and once they start moving, clues start being given away.