Below you can find some NATO markings
This may be useful to know these standard NATO signs used during urban combat. Especially building clear and medic ones. Bud booby trap mark can also save your live.
These markings can be fabricated from any available material.
These marks would be seen in building windows during combat operation. These are portable ones so after operation troops probably remove signs.
But inside buildings marks advised to be made with at least with semi-permanent writing materials which are not affected by moisture. So after troops leave area these signs remain for a while especially in upper left side of doors or other places that will be possible to spot them during passage through entry. So why not to use this information.
Some marking devices are described here. Military claims chalk is most common as has a lot of advantages. It is light and easily obtained.
Wolf tail is also developing as marking device.