Easter egg. OK, hear me out. I thought of hiding Easter eggs for children to find. Then I realised Easter egg is the common term for things video game developers hide in video games for a player to discover. It can be anything, usually it's an allusion, joke or some other object embedded in a video game by the developers for a player to find. They are hidden, but not so secretively that you won't find it, kind of like hiding Easter eggs for kids, they're intended to be found.
This screenshot is taken at the top of a parody of the Golden Gate Bridge in Grand Theft Auto: San Andreas (2004).
So I've checked, hoping that this term has spread beyond just video games, and apparently it has, though to what extent I'm not sure:
Easter egg (media)
It mentions Easter eggs in comics and video.
Pixar Animation Studios are known for their inclusion of multiple
Easter eggs in their films.
(At source above given, under "Popular culture")
I have no idea how familiar this term is to people in general or to what extent it's achieved a generic meaning outside its original meaning. I suppose many older people or those without much interest in software or video games won't be familiar with this use.
However, an advantage is that even if unfamiliar with this usage, an Easter egg hunt is popularly known enough that "Easter egg" can easily be used as a metaphor for something hidden and intended to be found. I know you didn't ask for a metaphor, just saying. Also, it's not a single word.