The other one was manufacturing and engineering teams 'back home' would scrawl the Kilroy on parts, like while ships or tanks were being assembled, that would otherwise be inaccessible - which meant that when that thing was hit, or taken apart for maintenance closer to the front, Kilroy was like, inside the sealed-up wall or at the bottom of the engine compartment.
In both your example and this one - both growing the myth that no matter where you went, Kilroy had been there first.
According to my grandpa, it was a myth that they used to feed the new guys and green squads, like a Santa myth, and putting on "genuine belief" in the Kilroy myth was as much of a running joke as the myth itself was. He claimed servicemen were also constantly trying to get commanding officers to unwittingly participate, by doing stuff like submitting paperwork signed Kilroy or that referenced him already being somewhere when troops liberated it - in the hopes that report or news tidbit would be one that COs shared as announcements.