To save us both time and trouble, here is a checklist. It isn't really hidden. In a movie, for example, obvious references to other movies aren't Easter Eggs, even if some people don't pick up on the references. Corporate logos and references to other famous people or products are in tons of things and aren't Eggs. It was accidental.
Eggs are there on purpose.
It is symbolism, foreshadowing, or any other literary technique like this. Storytellers use these all the time to tell a good story, and they certainly don't count as Eggs. It fits in with the context of the work in question. Eggs stand out as being different from their surroundings. It is a plot inconsistency, or just some wierdness you can't explain.