The microcontroller that also handles Unison is also powered through the USB port in addition to the wall wart.
It's done that way because if it would only be powered through the wall wart and you switched Mimir off, the host (PC, Mac, streamer, whatever) would lose contact to Mimir and Mimir would disappear from the connected devices, including that annoying USB disconnect sound on Windows being triggered and everything. Some player softwares even completely forget that you ever had the DAC selected as the output target when it goes away even just temporarily. That's not great. And so we decided to keep it powered whenever possible while Unison is connected to a powered-up host.