root@work:/# sudo apt-get update
Hit:1 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu xenial InRelease
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:3 http://nyc2.mirrors.digitalocean.com/ubuntu xenial InRelease
Hit:4 http://nyc2.mirrors.digitalocean.com/ubuntu xenial-updates InRelease
Hit:6 http://nyc2.mirrors.digitalocean.com/ubuntu xenial-backports InRelease
Get:5 http://ag-projects.com/ubuntu xenial InRelease [7,109 B]
Err:5 http://ag-projects.com/ubuntu xenial InRelease
The following signatures couldn't be verified because the public key  is not available: NO_PUBKEY F74046C316D8F9F5
Fetched 109 kB in 0s (181 kB/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://ag-projects.com/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F74046C316D8F9F5
W: Failed to fetch http://ag-projects.com/ubuntu/dists/xenial/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F74046C316D8F9F5
W: Some index files failed to download. They have been ignored, or old ones used instead.
*****************************

root@work:/# sudo apt-key list
/etc/apt/trusted.gpg
--------------------
pub   1024D/437D05B5 2004-09-12
uid  Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
sub   2048g/79164387 2004-09-12
pub   4096R/C0B21F32 2012-05-11
uid   Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>
pub   4096R/EFE21092 2012-05-11
uid   Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>
pub   1024D/FBB75451 2004-12-30
uid   Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>
pub   1024D/163A0DF5 2007-05-24
uid   AG Projects Debian Package Signing Key <support@ag-projects.com>
sub   2048g/9565A2F3 2007-05-24
pub   2048R/9CA27E41 2018-02-02
uid   John ATEL (Newkey) <info@newconcepttechnologies.com>
sub   2048R/36CFB265 2018-02-02
sub   2048R/9934ADF8 2018-02-02
/etc/apt/trusted.gpg.d/nilarimogard_ubuntu_webupd8.gpg
------------------------------------------------------
pub   1024R/4C9D234C 2010-01-20
uid   Launchpad webupd8