Those are the rags that were clogging the drain. The night cleaners get stuff sucked up in to hoods all the time. It gets really bad when its a water wash hood.
Heres a hat that was sucked up and was stuck to a fire sprinkler inside the duct:
Mikey dirty jobs are easy to find, the problem is no one will pay to get them cleaned, that's why I don't have similar pictures to post, that's also why they are easy to find. There’s always someone willing to not clean it for less than I would charge to clean it.