Why is Steam showing an Xbox 360 controller that doesn't exist?

My (native Linux) Steam client is showing an Xbox 360 controller, though I don't have one, and none have been in my home (and it's a desktop, so seeing it outside can be ruled out). I tried listing USB devices, detaching devices, looking in the paired Bluetooth devices, and it persists. What I found in a Google search is a case of stale device drivers in Windows (after it had the controller attached, do it's not the case here). What can cause this?