Looks like a "wind egg" to me. It's just an egg that forms due to maybe a yolk particle, or a piece of tissue from inside the hen that get's treated as a yolk and then coated like one and pops out as a tiny egg. I have a bunch of them and they often don't have yolks.
What a great response! Thank you all for your help.
They are the cutest little things, but I guess I'll have to crack them open to see if they have yolks.
'wind eggs' ... does anyone know why they are called that? Do the hens think they're breaking wind??