Or shortly after I left. I need one of those things that detects earthquakes, and put it on the desk. A vibration could be waking it up. Without buying anything, you could just switch the input on the USB switch, oh wait the Mac mini is off. In that case, unplug it.
If it still gets woke up, a vibration isn’t doing it. But then wouldn’t it mean a hacker is?
You’d need one that logs, or that the camera can see.