From the Times-Pic:

Search teams from as far away as Canada ride airboats through receding waters in a parish that’s become a virtual ghost town except for the rescue workers. They conduct house-to-house searches for anyone who might still be alive. When they finish searching a building, they leave a spray-painted marker to indicate whether anyone was found.
An “X” means the house was empty. A number indicates how many bodies were discovered. On one house, just a block away from the Bell South building where evacuees are being taken for decontamination before being transported to the slip, a blue-pained “6” tells the gruesome tale of what became of those inside.