The markings of this door were made by search and rescue teams in the immediate aftermath of the September 11, 2001 attacks on the World Trade Center. The code indicates that the search occured on September 13, 2001. The "X" in a box represent that the conditions at that time made this a dangerous area. The other symbols represent that the search of the area was undertaken by the US Federal Emergency Management Agency FEMA, Massachusetts task force.
Submitted by @lampbane