Kilroy was here
Kilroy was here is an American symbol that became popular during World War II, typically seen in graffiti. Its origin is debated, but the phrase and the distinctive accompanying doodle became associated with GIs in the 1940s: a bald-headed man (sometimes depicted as having a few hairs) with a prominent nose peeking over a wall with his fingers clutching the wall.
"Kilroy" was the American equivalent of the Australian Foo was here which originated during World War I. "Mr Chad" or just "Chad" was the version that became popular in the United Kingdom. The character of Chad may have been derived from a British cartoonist in 1938, possibly pre-dating "Kilroy was here".