A white square with a question mark, often used to represent unknown flags.
Own work based on: Flag of None (square).svg

Own work based on: White square with question mark.png
Rainer Zenz, Huhsunqu, Zscout370  and Ssolbergj (Flag of None (square).svg)Rafał Pocztarski (White square with question mark.png)OmegaFallon (This file)