The Legend of "Kilroy was here" was started inadvertently by a shipyard inspector during WWII named James J. Kilroy , who used the logo to indicate his inspection of riveting in the newly constructed troop ships was complete.
To the unfortunate troops outbound in those ships, however, he was a complete mystery ... all they knew for sure was that he had "been there first". As a joke, they began placing the graffiti wherever they (the US forces) landed, claiming it was already there when they arrived.
Kilroy Was Here