I managed to get the following answer, seems reasonable enough?

"The first computer game to feature a secret area behind a waterfall is generally considered to be the 1978 text-based MUD (Multi-User Dungeon) created by Roy Trubshaw and Richard Bartle."

And the game serie Thief started the "money in toilets" if I remember correctly. :P

Pretty sure that easter egg, started in real life, so if you get the idea of demanding copyrights give the royalties to mother nature perpetually.

https://en.wikipedia.org/wiki/Journey_Behind_the_Falls

probably had its start here (late 1800s).