This only happens if the ISP participates in a FBL
ISP (Internet Service Provider) Feedback Loops (FBLs) are mechanisms that allow ISP users (end subscribers) to click on the “report spam” or “this is spam” button in their email client, which tells their ISP that they find a given email message to be unwanted.
After an end subscriber clicks on the “report spam” or “this is spam” button in the user interface of a given email client, an “FBL complaint” is sent back to Salesforce Marketing Cloud for processing. Upon receipt of this FBL complaint, the recipient who lodged the complaint will be unsubscribed from mailings from that client. The complaint is logged and the recipient will show up in send stats as a complaint and as an unsubscribe.
You could review the _complaint dataview to see which has been reported