I just ran into this issue and I think I can give a better description. There are a bunch of pdfs that are synced to my web library but the local version of them has been deleted. If I double click on the files, the zotero client is able to pull them from the server but they aren't present locally and if I say, try to tell zotero to show the file or retrieve metadata from pdf I'm given an error that zotero can't locate the file.

In my use case, I deliberately deleted the local copies to save on disk space but there are a bunch of files that I forgot to process for metadata (i.e. create a parent item from the bare pdf). I'd like to do this in mass rather than double click each and then have to close it in my pdf reader but I don't see any way to accomplish this.