Good idea. Checked the logs on a high traffic website. And indeed, there are GET requests with the _sm_byp parameter. No referrers. But the user agent is "Media

Good idea. Checked the logs on a high traffic website. And indeed, there are GET requests with the _sm_byp parameter. No referrers. But the user agent is "Mediapartners-Google". Also the IP is from Google. So either a bot from google added the parameter on its own, or it found it in the wild.