There's no way to search a catalog for items unknown to the catalog.
The status indicator (little circle that appears to the right of the thumbnail filename) tells you visually what items are in a catalog, are in a catalog but aren't online, or are not in any currently searched catalog. The colors are:
Green: online and in one or more catalogs currently selected for searching.
Yellow: offline or missing but known to one or more catalogs currently selected for searching.
Hollow: unknown to any catalogs currently selected for searching.
It would be possible to add a feature that would select unknown images, but it wouldn't be fast like a catalog query would be. That said, if the folder doesn't have many tens of thousands of images in it, the performance would be good.
-Kirk