I'm not even getting that far - it doesn't exist there to set!

When it disappears, it doesn't just disappear from view - it disappears from the list of things available to hide/show - as if no such thing ever existed.

I think it's survived an hour now. We'll see if it lasts 24. I bet it doesn't!

Went to bed with icon still showing in the tray.

Got up this morning to find that not only has the computer rebooted during the night to finish installing updates, without my OK to do this (files and browser sessions I wanted were still open), but immediately after notifying me of this - in fact, before I'd even had time to finish reading what was done - the update progress notification popup AND the associated icon both disappeared.

Icon is no longer in the system tray, and not available to put back in the tray.

As predicted, did not last 24 hours.