I'm a fire fighter, and I know that for search and rescue, you use an X system to mark a room. For example, if a search is in progress in a room, you would put a slash. Then after you complete your search, you'd make another slash to make an X.

This is interesting, however, I doubt if there was a building fire there, they would be looking for anyone. Perhaps the marking is old...
Or as someone else suggested, the rooms had been stripped of copper fittings etc.