During the Nazi-occupation of Lviv from 1941 to 1944, the Germans established bomb shelters in basements around the city. To this day we can still see some of the signage, namely, the letters “NA” and an arrow pointing to the shelter. According to one source, “NA” (short for “Notausgang”) marked emergency exits.
Most of the examples I found are on modernist buildings built in the interwar period. Maybe these were chosen specifically because they were newer buildings and therefore structurally more sound than the older Austrian-era buildings.
See also Bomb Shelter Covers in Vienna.