Unknown icon appears in MacBook Pro menu bar running Sequoia

I have attached a screenshot of the menu bar on my MacBook Pro, running Sequoia 15.7.7. On occasion, an icon appears in the menu bar - in my screenshot I added a red arrow pointing to the icon. The icon somewhat resembles a CD, but there is no CD reader attached to my MacBook. I have checked the Mac documentation and several web sites that claim to document every icon used on the menu bar, and I can't find this icon anywhere. The icon only appears occasionally, and I can't tell what app might have caused it to appear. Nothing happens when I left- or right-click on the icon, and there is no tool tip when I hover to tell me what the icon might be.

What is this icon? What app is running that generates this icon?