When I started recently MB a popup appeared:
"The follwoing files no longer exists at their expected location - remove them from the library?"
I clicked "NO" button to check the problem at a later time.
Now I have time to check the situation.
But when I launch MB again the popup does not appear again.
It seems to me that MB show conflicts only 1 time.
How can I tell MB to re-check the library for non-existing files?