Lightroom: Disappearing folders and images

Can you see the missing photos in the All Photographs collection in the Catalog section near the top of the left hand panel in Library or are they at least counted as being there in the All Photographs collection even though you can't see them?

Is there something different about the photos that you see vs don't see, their format (raw vs jpg), the camera they are from, the hard-drive where they are located, to understand if you might have some filtering set somewhere.

Are the folders for the photos listed on the lefthand side in Library but if you click into one of those folders, no photos are shown, or are the folders with the missing photos not even listed?

Images have disappearing from random folders in multiple catalogues since upgrading to LR 2015.4. The images are still where they should be and are visible in Exlorer.