So sorry to hear you lost one, Nifty. I found this thread because I lost a hen last night too. I found her this morning in a nest box. I saw her in there last night and thought she was going broody, but this morning, she was dead. No trauma or injury evident on her body. But the area around her vent was caked with what looked like dried egg yolk - although the actual vent looked clean. I've had trouble with her laying soft shelled eggs, so I imagine it may be related to her egg making somehow. I did my own amateur necropsy of sorts - I opened her up and her abdomen was filled with what looks like yolk. Lots of runny yellow stuff in there.
She was a Golden Pencilled Hamburg and the only other chicken I've lost was another Golden Pencilled Hamburg about 6 months ago. That one was fine at 4PM, sitting hunched in a corner around 5PM, and dead by 6:30PM. I had brought her inside in a crate when I saw her hunched down, and she just kept going downhill (getting more and more lathargic) and eventually died in a matter of about an hour and a half later. She was the first that I lost and I decided to wait and see if any others started looking puny. They never did, so I chalked it up to something this hen ate, or something that was just wrong with her genetically or the like.
I did a search this morning and found your thread. I'll be interested to learn what you find out.
Sorry for the long post!
Penny