She relates how she ran into a reference on a message board to "a glimpse behind the green door," which she said is "intelligence slang for restricted information and locations," and then later the same day overheard the 1956 pop song "Green Door," which was part of the soundtrack of a movie her housemate was watching. Looking up the song on Wikipedia, she discovered that it may have been inspired by the 1906 O. Henry story "The Green Door," about a man named, of all things, Rudolf Steiner. Wikipedia opines that "O. Henry uses the eponymous green door as a symbol for everyday adventures which he encourages us to seek out," and the emailer said that my recent exploration of an abandoned restaurant (described in "Owl time, and cold noodles") seemed to be an example of that.
She can't have known, because I didn't mention it in my post, that in order to enter that restaurant I had had to pass through a literal green door.
My own syncs, of course, have not been about the Green Door but rather about the Owl Door. Nevertheless, I thought I might as well look up the O. Henry story and read it. When I ran a Google search on o henry the green door, one of the first results was this:
Apparently, Owl Eyes -- named after the Great Gatsby character -- is a website that has various literary texts with annotations. I had never heard of it until today.
My own first association when I hear "green door" is Bilbo Baggins. In the first chapter of The Hobbit, Gandalf "scratched a queer sign on the hobbit’s beautiful green front-door," later explaining that the sign meant "Burglar wants a good job, plenty of excitement and reasonable reward."
I suppose that by "breaking into" the abandoned restaurant, and even pilfering a few figs, I have officially become a burglar.
(Note: Original post modified after discovering that the anonymous emailer is actually female.)