One of my desktop box I recently got this message when I wanted to update it via apt :

Fetched 1448 B in 8s (180 B/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://xxxxxxx ./ Release: The following signatures couldn't be verified because
the public key is not available: NO_PUBKEY 0000000000

Solution for it is very straight forward but lets write it down here since I always forgot how to do this. Here it is:

gpg --keyserver --recv-keys keyId
gpg -a --export keyId | sudo apt-key add -

Of course you will need to replace keyId with a real GPG key which is missing/uknown to APT.

Additiona resources - SecureApt Debian wiki page