I've a pretty good idea what's inside a PC, having built my own server a year or so ago, and I can't
begin to imagine what that might be from. I suppose it could be something evaporating off the CPU (which gets hot even with the fan), or off something else hot. Or maybe your coffee is too strong
If it's around the ioniser, that suggests it's electrically charged (swapping my IT hat for my physicist's one). I suppose it could be dust kicked out by one of the fans inside the PC. They are made from plastic, so would tend to build up a static charge.
Alex