Ubuntu – Ubuntu GNOME 16.04 autoupdates regardless of settings

This is driving me nuts – I can't figure out how to disable autoupdates! In 15.10 I could turn off auto updates and I'd only get a notification if I had manually done a sudo apt-get update without actually upgrading, or it might tell me to restart after I've done a sudo apt-get dist-upgrade if it warranted a restart – both of which were still annoying.

In 16.04 I have the same settings, but I keep getting notifications out of the blue that I have software to update. I'm often tethering from my phone with a metered data plan and don't want it to be checking for updates in the background, nor do I want it to pop up notifications at random times – I'm usually pretty good about manually updating.

Here are my settings in the Software & Updates app:

Ideally I could turn off the "Display immediately" and "Display every two weeks" options as well, but regardless it shouldn't be checking for updates at all, based on the "Never" option.

I've also gone as far as modifying /etc/cron.daily/apt-compat and /etc/cron.daily.aptitude to put an exit 0 at the top so they shouldn't be running anything.

My /etc/apt/apt.conf.d/10periodic file looks like this:

APT::Periodic::Update-Package-Lists "0";
APT::Periodic::Download-Upgradeable-Packages "0";
APT::Periodic::AutocleanInterval "0";

Edit

Now I'm just taking stabs in the dark:

• I've added an exit 0 at the top of /usr/lib/update-notifier/update-motd-updates-available
• I commented out the "${distro_id}:${distro_codename}-security" line under Unattended-Upgrade::Allowed-Origins in the /etc/apt/apt.conf.d/50unattended-upgrades file.

Hopefully one of those changes will cause it to leave me alone…only time will tell.

Edit, Part Deux

A day later, after making those other changes, I got yet another notification…so that didn't really seem to help. I'm really stumped and I don't understand why the built-in settings don't do anything.