chattr +a -V

Laughing at myself: switching between tabs in multi-gnome-terminal, I ran `apt-get update && apt-get upgrade` from within a directory I had removed, and got that error message. The clue to the solution and what caused the problem was 'cannot access parent directories: No such file or directory'. A simple `cd ..` did the trick.

The latest Debian Weekly News has links to solving the GPG problem `apt-get update` displayed after the first of the year ('The following signatures couldn't be verified because the public key is not available', etc.): Re: GPG error on apt-get update.