Ever since I hit the mark for review queue privileges on new questions and new answers to old questions, I've noticed something odd. If only one queue has a new post to review, the review queue icon shows as empty. I have to actively click on and enter the queue to see the alert. The red "active" circle only shows up when both queues have multiple posts to be reviewed. Is this a bug, or is this normal?
You might want to read Experiment: Review-needed indicator logic for sites that sometimes have empty queues, especially this answer:
Several people have noticed that sometimes the indicator doesn't light up even when there are tasks in one or more queue. After you click on the review queue icon, you get an hour-long grace period from being notified about new items. But the reports generally take that behavior into account. It looks like there's another reason for the occasional false negative: caching. So there's a potential delay of a few minutes between when a task enters review and when the indicator might first light up. When someone clicks the review icon, the number of pending tasks for each queue is recalculated and the grace period starts for that user.
And What criteria should we use to determine which review queue indicator a site will have?, which shows that the threshold for activation is "maximum median tasks per hour <= 2", meaning that more than 2 tasks show the indicator.
All in all the review queue indicator is just a bit weird at times. No bug, status-bydesign