I haven't finished this game. From the reviews I have read, that may not be possible. But the reviews and threads listed make no mention of what I found. Invisible Bad EGGs!

They are found in your own Pokémon Storage. I have found three so far.

The way I found them was that when I was about to deposit a Pokémon into my storage box 6, I noticed that the number of Pokémon listed was more than the amount I put into the box. So I started placing the hand over blank spots in the box and selecting them until I found something.

I haven't checked all my boxes, but I will. I am currently trying to get one to hatch.

I also am unsure of when these start showing up, as the description given is that they were found by the day care man. This may only happen after you actually talk to the guy.

I own the original Pokémon Gold, and don't remember anything like this in it. I haven't played Pokémon Fire Red or the original Pokémon Shiny Gold (hack of Pokémon Fire Red), so I don't know if this is something that was in either the original game or first hack that got messed with for this hack.

Mystery eggs are usually unexpected glitches and tend to be common and hacks. But do not hatch it. It will either freeze the game and corrupt game or freeze and not corrupt. Usually it is the first.