As I mentioned in an earlier post, the message/popup doesn’t stay there indefinitely:

There would be a possibility if you weren't present when that message was displayed, it may not have updated. But I don't believe it only asks once.

So it is very hard to say why you didn’t see it to authorise the update.

In very recent versions of avast (12.x.xxxx onwards I believe) the Program Update setting was set to Automatic Update by default. Personally I don’t like that as I want to control when a program update occurs so I change it back. Having it set back to Ask when an update is available, it should honour that option for future updates.