I agree with socratis. There are some issues with shared folders. In my Windows 7, 8.1 and 10 guests, \\VBoxSrv is empty but I do have an E: drive which is the shared folder. The contents are there and I can access them but the drive is listed as "Disconnected Network Drive".
Shared Folder settings are Auto-Mount, Permanent.
Edit: I forgot to mention, if I remove the shared folder from the guests, the E: drive remains in the Windows guests and is still labeled "Disconnected Network Drive". And I don't know how to get rid of it. If I try and disconnect it, I get an error dialog saying it doesn't exit.
From my 8.1 guest. W7 and W10 look the same.

- Screen Shot 2015-02-14 at 7.33.14 AM.png (124.85 KiB) Viewed 15723 times
Re-Edit: repaired W7 and W8.1 shared folders by removing the folders while the guests were open. Restarted the guests. Rolled back to the 4.3.20 GAs. Re-added the shared folders.