This afternoon, I was puzzled when I discovered that every time I lifted my mouse off the desk, the volume indicator would flash on the screen for a moment. It was pretty irritating. I wondered if i had turned on some accessibility feature. Or if some app had gone haywire. Or if I had malware. I googled and found a number of pages reporting similar behavior, but nothing diagnostic.
I spent some little time trying to diagnose the behavior. It seemed perfectly repeatable. Lift mouse, icon appears. I checked System Preferences for the mouse and accessibility without finding anything. I tried quitting apps. I tried rebooting. But the behavior persisted. It was perfectly reliable: lift the mouse and the volume icon appears.
I finally went to talk to the IT staff to see if they'd seen anything like that. George said he'd seen a computer doing something like that yesterday. But he hadn't figured it out.
I sat back down at my computer and looked at the screen. When I lifted the mouse, there was the volume indicator looking like it was thinking about changing. But what could be changing it? Then I remembered that I'm using one of the Matias keyboards and they have a dial for the volume on the back. And the dial was pressed up against the base of my monitor. Doh! When I moved the keyboard so that the dial wasn't touching the base, the behavior vanished.
Maybe this post will help someone else figure it out when it happens to them.