Shell-less egg. First egg?

I got a shell-less egg yesterday. I found it on top on the brooder roof inside the coop. I have several girls laying now, but they use their nest boxes. Well....the silkies use a different one then the rest cause its on the floor. The rest of the girls use 2 of the 3 boxes on the wall (nobody like box #3 ) but all in boxes! I got 2 eggs from a new layer this past week....but in the box!

I have 2 other girls who are about ready to start laying and they both hang out on top of the brooder. I found the shell-less egg in the poo pile on top of the brooder. So.....is it safe to assume it was one of these 2 who laid it? I'm thinking maybe she thought she had to poo since she's never laid an egg before?

 

Thanks. They have oyster shell in a small feeder by their regular feed. Its the odd location that has me thinking its a first egg. All my other hens laid normal first eggs, in the boxes.