Mac mail - brief red banner

I'm on a Macbook Air with Monterey (12.0.1) and when I use Mac Mail I've sometimes noticed a small band brief flash of red across the top of my inbox. This is not a screen glitch. It's like something in Mail that's trying to alert me to something, but it goes away quickly. I'm wondering if it could be something like an alert that I've lost connection to the internet or something... but I can find nothing about this anywhere on the internet.

I've attached a screenshot that SIMULATES what I've seen. Like I said, I can't screencapture it fast enough. (The black is just to block out what my emails are.)