18.04 – How to Permanently Disable Software Updater Popup

I know this subject has been discussed many times.
I have applied almost every fix that I can find on the internet, but every time my Ubuntu18.04 systems starts I get a Software Updater popup.

I have an Ubuntu 18.04 PC running a single 'Kiosk' application with a touch screen. I'm also using Overlayroot to create a Read Only operating system. This system does not need updates – ever.

in the GUI Software & Updates utility I have set 'check for updates' to Never. I have turned off all Update Notifications, I have even now uninstalled Unattended Updates. But every time my PC starts about 1 minute later the Software Updater pops up with 'Updated software is available for this computer'.

I assume that somehow when I was building the system, and had it in Read/Write mode, a potential update, found itself onto the hard drive and now sits there notifying me of its existence every time I boot the system in Read only mode – that is to say that the notification is permanently stored in the Read Only drive. It even pops up when there is no network connection. However, I put the system back into Read Write mode and cancelled the notification, but still it persists.

Where is this notification coming from? Is there some file I need to delete?