Secrets in Plain Sight: 40 Little-Known Codes All Around Us

In IT support, the code "ID10T" is used humorously to refer to computer errors (pronounced ID ten T) to indicate that the error is the result of the user's stupidity.

Printers print an invisible secret code called Machine Identification Code on every single page you print for the lawmakers to track date, time and serial number of the printer that prints it.

Violets represented a secret code between lesbians, identifying them among heterosexual peers, and in films. The tradition originated from Sappho’s poetry in 600 B.C.

Obituaries have long employed a secret code, a shorthand to mask causes of death that survivors found embarrassing. When HIV/AIDS began claiming lives in the 1980s, young men were said to have simply succumbed to "pneumonia." There is also a code for gay men that says they were "Unmarried" or "Confirmed bachelor." The codes “died at home” or “died suddenly” or simply “passed away” come out when there’s a drug overdose or a suicide.

The lamp posts in Central Park in Manhattan have secret codes on their base so that you won't get lost. The newer street lights have a code that is similar to C8202. C meaning center drive (W and E meaning West or east respectively). 82 meaning the closest street is 82 st. And the last 2 numbers meaning the pole in that area as it corresponds to a map.

Flippable yellow signs which you may sometimes in commonly in Hollywood and sometimes in other cities are secret-coded directions for actors to get to filming locations. They often feature insider jokes like: "Freezer Burn" signs pointing to the Captain America shoot (a reference to him being put on ice)

"Friend of Dorothy" was code popular in the 80's, which gay men used for secretly identifying each other. The higher ups in the Navy believed that there actually was some woman named Dorothy at the center of a massive ring of homosexual military personnel, so they launched an enormous and futile hunt for the elusive "Dorothy".

Tanker trucks transporting dangerous goods are labelled with UN numbers. There are lots of these numbers, but the useful ones are these. 1202 means diesel, 1203 is for gasoline, 1075 is for propane, 1267 is for crude, 1993 is for anything flammable,

Hobos have secret codes called the Hobo Code that was drafted at an 1889 Hobo Convention in St. Louis to help other hobos and also to warn them of dangers in the area. For example, their symbol for "kindhearted lady" is a cat.

The handkerchief code was common among homosexual males seeking casual sex and for BDSM during the 1970s. The color of the bandana and which pocket it's displayed in indicates what type of sex they're seeking.