A bag full of clothes.
So, the robbers came into the bank wearing painters clothes. Then they make everyone strip and put their clothes into bags and make them put on identical painters outfits, so as to confuse the coppers. In the end, they find the bags of clothes in the supply closet. Inside of the bags, there would only be enough sets of clothes for the victims, leaving the real bad guys wearing nothing but their skivvies. If they would have attempted to match the clothes to their owners, then they would have been able to narrow down the suspect pool quite a bit.
I mean, I suppose that it's possible that the robbers could have planted extra clothes in the bags... they DID think of everything else, but they never showed it. Not a gripe, per se. Just a thought I had as I was watching the film. They did make it a point to show that crime did turn out to be somewhat victimless, and none of the higher ups were really concerned with anyone taking the fall. More so they just wanted it to go away.