Certainly happening to me with the Mayor tab.
Have you tried cleaning your mouse? You might be accidentally clicking.
Good thought, but I'm using a touchscreen 
Not discerned a pattern yet, but it closes almost immediately, so that the unseen highlighted events are no longer highlighted when I re-view the mayor. Hardly a major bug, just an occasional nuisance.
Funnily enough, I've just had this in a different way: I was looking at one of the buildings in one of my towns and somehow must have engaged the Upgrade button.
Have you tried cleaning your mouse? You might be accidentally clicking.
I can't remember the last time I had to clean a mouse. All optical and micro switches these days. In any case it isn't hapening anywhere else.
