Here is a short explanation to generate a list of Lr images and a separate list of images in Explorer.
Simply paste both lists into Excel and use the Conditional Formatting / Find Duplicates to highlight matching entries.
The items not highlighted are missing from one list or the other.
So...here goes.... Ps. It is easier to do than describe....
Using ListView PlugIn from John Beardsworth.
In Lightroom go to the folder of interest and select all images (better to only select masters and not virtual).
Using John's plugin ... pick the option to export to Excel as CSV (because there is less formatting),
Close ListView. and back to Lr Grid view.
Select a single image in grid view.
Right Click and Show in Explorer.
Select all images in the folder (Ctrl A) and then simply click on the Copy Path button.
This places a list of all the paths of all the images in the clipboard.
Go back to the Excel sheet and paste the file names into column 2.
The column on the left are the files from lightroom. The column on the right are from Explorer.
In Excel, with the Home tab selected ... click on Conditional Formatting.
Select both columns and then use Conditional Formatting... to highlight Duplicate values.
All cells highlighted have duplicates in the other column.
Any cells not highlighted are missing ...
ie. If in the left column....they are in Lr but not on the disk.
if not highlighted on the second column then they are in Explorer but not in Lr.
Also, if you are handy with Excel then you can get far more sophisticated ...
Some tidy ups are best done using Grid view or similar. but sometimes having a list can simplify ... especially if dealing with large numbers.
Be careful when creating your Lr list to include only real images (ie use the filter bar to exclude virtual).
Post a query here if anything needs clarification or send me a private message thru the Forum.
Regards.
Ps. I did a short video of this but was unable to post to the forum.... pity...