Lightroom :: 3.6 Wants To Import Non-existent Pictures?

Lightroom 3.6 has developed a strange problem ever since I tried to import seven pictures (canon RAW) directly from a CF card (rather than using Windows and then synchronising folders which is what I usually do). use of a CF card may be a red herring. Also, I’m using Windows 7 SP1.
 
To cut a long story short, if, in Lightroom Library, I now select “My pictures” (the top level folder) I can see all the folders and pictures that have previously been correctly catalogued and synced in Lightroom. I right click and select “Synchronize folder”. I would expect nothing to happen, because all folders have been synced, BUT, after a delay I get “Import new photos (644)”. There are no photos to import, and therefore, if I then let Lightroom try to import them, I get the error message “No photos were found to import”.
 
Every time I want to synchronise, I get the same problem.
 
I tried using a backup LRcat file. In this case the same thing happens, but it wants to import over 700 pictures. This would be partly correct as they will be about 100 genuine pictures to import due to the LRcat file genuinely missing some of my more recent photos.
 
Also, if I add a new folder and picture files in Windows and then open Lightroom to synchronise, it will want to import the 644 plus the correct number of new photos in the new folder. It then imports the genuine pictures. If I then try to resynchronise, I’m back to the 644 false ones.
 
This tells me that Lightroom has decided it needs to import 644 false pictures, plus any new genuine ones.
 
How can I get rid of the 644?