Maybe look in Steam -> Settings -> Downloads + Cloud -> Steam Library Folders?
It shows only one, D:\Program Files (x86)\Steam - which is correct.
It seems I'll just have to re-download the game tonight between 00:00 and 08:00 (when I can download freely - I have a 250 GB monthly cap the rest of the time).
I used "Delete Local Game Content" then choose to start the re-download. It asked which directory, I choose D:. I have paused the download until midnight, but at least the Local Files tab now correctly shows "Disk usage: 6737 MB on drive J:"