Hi, TMW, I've had exactly the same a year or so back, again cameras inserted etc, nothing found. Possible causes were upped training/dehydration and small stones that got passed., mine was also accompanied by soreness and pain from the kidney area into my groin. I drink more now and it's settled down.
pipwolf, just my experience, but the 857s really didn't suit me at all. constant tired legs, hurty feet etc. I tried to stick with them but it got to the stage where I really hated going running in them. every run felt like a marathon.
I really hope you have a better time. Got Mizunnos now and they are brill.