I have a case that has a lot of tiny lil vent holes on the sides. When I open it after say, a fortnight of use, I see that along each hole, there is a small dust spot the size of the hole, along the case. I gently blow the dust away using a vacuum cleaner at the lowest air setting, taking care to keep the nozzle at least 2 feet away. I guess some or more of the dust that is sucked in from the fans is retained within the case.