Looks like a "breast blister" to me. How are their roosts? Are they wide enough so they don't end up putting too much pressure on one spot? Most mine sleep on roosts 4 inches wide or on the top of the hutches which are flat. .
I only found that on one hen. But they have alot of places to roost. They have thick roosts. But upclose it looks just like a belly button. You cant see the hole cause its got dried poop covering it.