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.
Urban SHTF signal set
There is a big crisis of public services including information exchange in all SHTF situation. Set proposed below, allows you to keep some information exchange abilities.
This is not global system but in crisis situation any information can be helpful.
Depend of nature of cataclysm most important is to attract rescue groups. So chemical lights and infrared markers are good and easy choice. Big HELP sprayed on building roof or parking place can be also helpful. Duct tape can do the same.
This is good to stick a sheet of paper on destroyed building which used to be your permanent stay place. Put your name, date, reason of displacement, number of persons their condition and direction of movement. So permanent marker and field notepad is perfect.
Use duct tape and or double-sided tape to stick them to surface.
Drawing chalk is perfect for markings on rough and absorbent surfaces.
Try to know some marks recognized by most peoples in the world to share information or be able to read them for your benefit here.
These are low tech items but in SHTF situation less technology is more.
After you gave signals to rescuers try to find some resources. Read what tools are most effective in city scavenge here.
Any other ideas of affordable and handy signal devices?