I created a brand new catalog and proceeded with a scan (just one folder path on a hard-wired external drive). When it finished, the scan showed 8890 images. I did nothing else to that catalog after the scan, I didn't even browse it.
The next day I was looking at the ‘Sync Catalogs’ feature. I had never used it before so I selected the new catalog a ran the sync just to see how it worked. When it finished I was surprised to see it found 13 new images that were previously not scanned, for a total of 8903.
The thing is, between the initial scan yesterday and the sync today, I never touched that archive. In fact I haven’t added to it or modified it or even looked at it in long time. I had a look at the ‘new’ files it found, and they were definitely not new.
No idea how those files could have been missed in the first place.