Thank you for the good advice, everyone! I will redouble my efforts using some of the tips here.
I went out to check periodically today and noticed the most likely suspect, Bridget, pacing around on the other side of our fence. There is a big gap in that fence that just leads to another part of our land, so the chickens are free to go there if they wish, but I have never seen any of them actually do it until now. Behind the area she went is a massive pile of broken pallets which would be literally perfect for her to evade detection and predation. It would also be near impossible for me to find the nest without risking injury to myself or breaking all the eggs.
I also noticed a little parting in her breast feathers. Couldn't see a bald spot but it didn't look normal either, so I'm definitely suspicious. Bridget went broody after hearing her sister, Henrietta, hatch chicks. Henrietta made a secret nest and disappeared on us, only to be found days away from hatch. When Bridget went broody, I chucked her out of the nest multiple times in the day into shady areas so she could cool down and she soon stopped sitting in the nest boxes. I thought I had won but it seems I may have been thwarted and she has initiated plan b.
Tomorrow I'm planning a stakeout: Operation Stake and Eggs. I'll go out during Bridget's normal laying window and do some gardening near-ish to the gap in the fence. If she disappears, I'll follow at a distance and see what she gets up to.
Not sure what I want to do about it if I can't reach the nest. I might just let nature take its course and if Bridget disappears, I'll build another broody coop for the inevitable hatch three weeks down the line. The joys of free ranging!