The magic egg, in both placement and probability is impossible.
But rogue Facehuggers that went elsewhere before they realised that there were viable hosts right next door is not.
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
It's my personal interpretation that the opening of A3 is the dream that Ripley later speaks of to Clemens.
But realistically, there wouldn't have been an egg- otherwise it would have been found.
