My normal routine with eggs that don't hatch is to hold them up to my ear and tap on them with my fingernail. Then I recandle, then I float test them. If I still have nothing, I pip open the end of the egg and look inside with a flashflight, then open the egg and see what happened.
A couple of hatches ago, I did the tap and float test and nothing on a few eggs (shipped eggs are brutal). I was depressed with the idea of seeing another fully developed dead chick, so I just pulled the eggs out and put them in a carton on the table. The next day, I started picking up the eggs and putting them in a trash bag to dispose of them (they had been sitting on the table for about 36 hours.
I had put about a dozen bad eggs in the bag and thought I heard a very faint peep. (I have very good hearing). I started pulling the eggs back out of the trash bag and holding them up to my ear until I found the one making noise.
I opened the end of the egg and there was a chick, pipped internally but no external pip.
I helped it out of the egg. That was about three weeks ago, I now have a perfectly happy, healthy, very tiny little serama chick running around the brooder with the rest of the chicks. It's name of course is Miracle and it has a home for life.
Deb