I had just found it and came to post , thanks !
United States FEMA marking
In the United States, a different FEMA marking system is used on searched structures, as follows:
A single diagonal slash indicates that a search in the building is in progress. This is used to indicate searcher locations and to avoid duplication of the search effort.
An X inside a square means "Dangerous - Do Not Enter!"
An X with writing around it means "Search Completed", with the time (and the date if appropriate) written above the X, the team conducting the search written to the left side of the X, the results of the search (number of victims removed, number of dead, type of search such as primary or secondary) written below the X, and any additional information noted about the structure to the right of the X.
These x-codes are used in a variety of situations and were prolific (and adopted and modified by other agencies) during post-Katrina operations.