That is why I ask, I have been doing the beginning of 3-1 a few times to get some upgrades and in the middle settlement are lots of eggs but mostly white ones, some brown and some gold but never a rotten one, I guess the are indeed drops as orderoftheflame says, are they drops from enemies later on as I have picked up every drop up to 3-1 and haven't gotten a rotten egg?
Rotten eggs don't appear until later on as far as i'm aware, level 5 onwards are the only areas I have had one drop from an enemy.