I caught these new marks on our building the other day. I've seen this symbol around for years on various buildings, the spray-painted square with the carefully drawn 'X' in it, corner to corner. I came to assume that it meant the building was going to be demolished and as they have appeared on our building, this seems obvious.
Nonetheless I decided to look up the symbol and find out exactly what it does mean. I found a link to a pdf entitled 'Building Markings CA OES / FEMA: Overview of the Federal Building Marking System'. It appears to be a powerpoint presentation for building inspectors, of marks to use where the possibility of damage to a building might exist.
With regard to the symbols on our building, I have extracted relevant pages and pasted below. Simply put and according to this powerpoint, the symbol seems to mean Bad. Personally, I think we were more of an OK, but...
