Thanks Pratyush,
That's what I had thought initially however in Windows explorer I normally have hidden items as shown. Doing checks from the command prompt running the usual commands like 'dir /A:DHSL' reveals other hidden folders and junction points, but the Enpass folder I created just does not exist outside of the application.
To help illustrate, attached are screenshots showing:
1. the folder and keyfile created in, and accessed from, Enpass.
2. the same parent folder trying to access using Windows explorer
3. the Error message when I copy the path from the Enpass keyfile dialog, and try pasting that path into the Windows Explorer address bar.
One possible explanation I can think of is Enpass has implemented some form of virtual folder mapping to hide the actual location of the keyfile.
Hopefully this makes the nature of the problem clearer.