This is an interesting thread, since I found a soft shelled egg just this morning. My hens don't lay many of them, maybe just three or four since they started laying in August, but every one of them have been laid outside of the nesting area. And all but one was already busted by the time I found it.
Maybe it has something to do with the cold winter weather?
But I'm not sure about that, because I have 2 panel heaters in their coop, which are rigged so that they come on when the thermostat hits 35 degrees and then turn off again when the thermostat hits 45 degrees. Also, I generally let them free range for an hour in the afternoon, but if it is freezing temp outside, then I don't let them go out that day.