We believe that a permanent zombie apocalypse is unlikely. Due to the amount of training and firepower possessed by the military, police, and militias of the world, the cavalry IS coming eventually. When they do, how do you safely notify them.
The simplest way is to write “Alive inside,” on or near the highest traffic area; with the number of alive listed. If looters or other dangerous individuals are an issue, double the number of people, and add “, armed, be careful.”
Also, if you have access to a flag, display it upside down; it’s an international sign of distress.
Source: Police officer.