I was a DVD programmer for just under 10 years. Some titles naturally allowed for more creativity in the menus. For one easter egg, I programmed a feature accessible after an up-up-down-down-left-right-enter navigation from a specific button. Pretty sure the marketing team "leaked" it on some forum for it to be discovered.
Any chance you remember what the title was?
Pretty sure it was Lupin, but couldn't tell you which of the plethora of them. I'd guess it was an Extras menu, but it went to a bit of the actor doing a goofy bit as the character not at all part of the actual content. Just the cast/crew having fun.
By nature of DVD menus and a remote with number keys, you can just add 6 to the number of buttons on the menu to type in the specific button on screen. Then again, not many consumer remotes had those.
It might have been discovered by people ripping the dvd. iirc the rip software detected the chapters
I was downplaying things when I said "pretty sure" as I know for a fact they did. Some people could have found the content while ripping, and some people could have found the extra buttons in a software player too.