A bird can inadvertently drop partially-shelled eggs, along with her poo at night. It happened to my novice egg layers, in the beginning. I wonder if it happens from time to time, especially when they are getting back into their egg-laying groove from a molt or broodiness or other. Also, recently I've had a more experienced hen start dropping rubber eggs, or sometimes they're called jelly eggs, from the roost at night. Those were shell-less, from what I could tell and the flock ate them. The flock also ate all the half-shelled roost eggs from the new layers. Maybe if a shell is too weak, the egg could be dropped at night, even from an older layer. An egg that breaks, while inside a hen, will kill her -- so it's a good thing when any weak-shelled egg is passed outside as soon as possible.
The question is, how old is your BR? And did she just get over a molt? If she's a beginning layer, it's VERY common to get roost eggs intermittently until their body gets the hang of it.
ETA: I just saw your post, above. So, she's more experienced layer and I'd look at making sure she's got constant access to crushed oyster shell, and always plenty of fresh clean water. If she looks healthy, eats well and her poop looks normal, you're probably looking at a bird that either is a touch broody (they will pluck out some of their breast/belly feathers) or she is going to molt, like you said.
