I haven't logged in here in months so if this was a thing before I left... But I currently have 1k csc in my shop and I'm not sure how I got it. There's nothing in the transaction page either.

CWs can be sold in user shops for CSC, so you probably had some in your shop that got bought. the sales history only goes back a certain amount of time (not sure exactly what the time frame is though) so they wouldn't show up if they were sold a long time ago.