I'm not sure when I first noticed this, but while using the context menu (right-clicking on a file) I noticed there were some blank options which is not a good sign. Testing them seemed to launch Acrobat so I went looking for the cause.
I found the solution on the Adobe site. It seems this behaviour is the result of having the Google-drive desktop application installed. I suspect it's more likely an issue with having the Adobe suite AND Google-drive installed but since the behaviour seems to affect Acrobat, the Adobe fix is to remove Google-Drive (I wonder if Google's fix is to remove the Adobe suite?).
Anyway I tried it and sure enough the invisible options are now gone. You can still use G-drive by going via the browser so it's not a major loss. I'm sure Google will fix this in a future version (if they haven't already) but for now paranoia levels are back to satisfactory. Incidentally when I logged back into the browser version of G-drive I had some invisible files there and it prompted me to see my 'new look' G-drive. I followed this link and after setting my view back to list-view, everything appeared back to normal.