Comment 9 for bug 1246364

Okay, I have identified the problem.

update-notifier supports a mode called "auto-launch". When it discovers new updates and auto-launch mode is enabled, it doesn't notify the user at all, but launches update-manager instead. If update-manager detects important security updates, it immediately displays a window interrupting whatever the user was doing. (I suppose some people might like this, but I find it very irritating.) On the other hand, if update-manager detects only unimportant updates, it remains minimized, which might mean it remains nearly or entirely invisible depending how the user's desktop displays minimized windows. It is very easy to miss on Xubuntu, and probably almost every other desktop other than Unity.

Some time between Ubuntu Raring and Saucy, somebody ripped out update-notifier's checks for whether the user's system has "auto-launch" mode enabled, forcing everyone into auto-launch mode instead. In one case, they even went so far as to leave this comment in place of the working code:

// everybody wants auto-launch mode

I'm guessing that the author of that comment assumed that the whole world uses Unity, which is clearly a mistake.