I've noticed that CK manipulates the NavMesh in the background, even without me bringing up the button. Simply placing XMarkers seems to make it mark unchanged navmesh as changed and saved. TES5Edit then says it's unchanged ITM and removes it again.

Anyway, I've added some XMarkers in Whiterun exterior 15, but it caused navmesh complaints in exterior 16 (where I'd actually modified a single triangle in front of the horse, making it preferred, days ago). I fixed that by re-Finalizing 16, and now I've got a complaint about a missing road marker (07077d) in exterior 17. I've not touched 17 yet, but I know that I will soon.

What the heck is a Road Marker? No mention in the CK wiki.

I tracked the base down (07077b) -- so that's pretty close to the original definition -- but there's no data there that helps. There are only 10 of them in the game.

The only other mention in Google is the same error message in French.

It's definitely not missing -- it's right there in the road, untouched.