I have seen that message from time to time, but like you I wasn't able to find anything running.

I purchased a 1TB USB drive that would occasionally give me the message. I contacted the makers and they said it was truly plug and play and not to worry about removing it at any time. There was no need to eject. But today, as a matter of course, I do eject the drive. If it says that it is being used, I just ignore the message.


BIAB for Windows Version 2019 (643)
Windows 10 (1909)