@Hairy
Existor is right about the alert disappearing, rendering it un-clickable. It reappears with a running total when nothing else enters the alert list.
Here-- it isn't fancy but you can see how this running total thing works, and how it behaves when it becomes interrupted. In each image, between completion of the Marines, the icon disappeared because of the 10-second fade out shown in the first image. I did this with 3 naked Rax.