That's gotta be it; "un" being update notifier. The image at that location is clearly the logo:
So I did the following:
mv /usr/share/icons/Yaru/scalable/status/un.svg /usr/share/icons/Yaru/scalable/status/un.unitednations.svg
ln -s /usr/share/icons/hicolor/48x48/apps/un-reboot.png /usr/share/icons/Yaru/scalable/status/un.svg
killall mate-panel
And we are good to go:
Thank you for your help in finding this!