What does the ∠ byobu status notification mean?

I'm using byobu on Ubuntu 24.10, and have a question about an icon / status notification that appears and disappears: it looks like an L, but the vertical part is "bent forwards" I think it's ∠. It had a red background and disappeared after a moment. What does that mean? And is there documentation somewhere for the other transiently-displayed icons and such? I can go through the menu to toggle status notifications on and off to see those, but some are intermittently shown and it's not clear what they mean.