Thanks! So I did a more thorough examination of all my chickens tonight. I looked at every vent, and at the rooster's bald spot skin. Here is what I found:
1. Mr. Edith (the rooster) had some reddish skin near his vent, though no egg bunches or crawling mites visible. There was a bit of dried poop on the feathers right above his vent. No crawling mites or egg bunches visible on his bald spot either
2. I swiped my finger along the roost and nest box and didn't see any red streaks that signify red mites
3. The other vents were similar to Mr. Edith. My two heaviest layers had firm, round vent areas with dried excrement on feathers and no visible mites. It was a little bit lacking in feathers though. My Buff Brahma was perfectly healthy everywhere, but I should mention that I have been treating her for scaly leg mites for the past month or so (bag balm on legs every other night)
*also, my RIR and heaviest layer has been laying very poor egg quality recently. They are thin, and pale with a dark spot on the very top :/
Any further ideas/theories are welcome!