I could be wrong, wouldn't be the first time, won't be the last.
So I looked into it some more and it appears there is some data is written but it's data dealing with the scrub operation, not your actual data unless there is of course an error. So it's not severe however it is writing data. I wish I could tell you how much data specifically as this would give some sense of the overall impact to the storage device. If you're running a SSD, I wouldn't worry about it (yes I retracted that part), but a USB Flash drive, I'd definitely not do a scrub once a day.