Actually, everybody's correct. NFD2004's response is applicable to FDNY and maybe other jurisdictions. In NYC you'll also sometimes see the letters "RO" (Roof Open) meaning that there are holes in the roof and the Roofman is not allowed to go there. The "X", as NFD2004 states, signifies that operations will be conducted as an exterior attack due to the extreme life hazards to members. In the case of a wide-area disaster, such as Katrina, the markings are used to show that the building, vehicle etc has already been searched, by whom, the date and if any bodies were found; I believe this may be a FEMA protocol based on the one established by FDNY in the 1970's (?). These markings were also used in 2001 during the search operations at the Trade Center