I get that filters hide and show images. Star ratings, colors as examples. The entire folder did disappear from the Catalog panel on the left, again. Attempting to reimport again shows me that LR still knows of it's existance and I have to uncheck the Suspected Duplicates box to reimport. Command-L (Mac) does nothing in this case. Searching for one of the file names brings up the original file but no the file I am looking for.
Maybe this might explain things. The hiddent folder is a temp that I set up for a print job as the job contains images from a few different folders. All the images are found under the same names on a different drive. I was able to import this folder and edit the images without unchecking the Suspected Duplicates box, which did surprise me. Might LR be unrecognizing the folder in question because the images are already in the Catalog? Even after reimporting with the Suspected Duplicates box unchecked, the folder still left the scene.