Simply viewing a photo in Develop starts with the preview but will then pull info from the camera raw cache. So I guess LR is recognizing the latest version of camera raw. There is no need to turn on automatically write to XMP. In fact if you ever had to restore your computer you would probably always rely on the latest catalog.
I haven't viewed any photos in the Develop module yet, only the Library and map modules. I don't mind LR recognising the latest version of ACR, but I don't want it writing the new info to XMP unless I actually edit that photo.
If I turn off 'automatically write to XMP', then won't LR write any changes I make only to the catalog, rather than to the XMP sidecars? I want LR to use XMP sidecars, but only to write to them when I change something.
Some users like to always write to XMP because they exchange edits with colleagues or clients using the sidecars. Some will also use Time Machine on the Mac for automatic backup versions or similar third party software on Windows. It’s economical to backup changes to those small sidecar files as the raw image files remain unchanged. So it’s a personal choice, but you have to accept that LR is constantly writing to the catalog and the sidecars where that option is selected in preferences. It’s just the way it was designed for non-destructive editing.
I don't think writing to the XMP sidecars when no changes are made should be needed for non-destructive editing though. Seems a bit strange to me.
Anyway, thanks for letting me know that what I was looking for is not possible. I'll turn off the 'automatically write to XMP' option and stick to Bridge for my editing and metadata entry, using LR just for the maps module.