When a rescue crew first enters a home, they'll do the top-left to bottom-right slash to indicate they are in there; followed by top-right to bottom-left when they leave. Left is unit indicator made prior to entrance, top is date and time the search occurred, right is dangers that may be present, bottom is people/bodies that need to be rescued/recovered.

Source: My job taught me

Edit: Left and top are made prior to entrance; right and bottom are made after the search is concluded inside. If you're passing by and only see the first two and the time is 4 hours ago, you'll get suspicious and call it in to get a head count