In the interstitial shown to users when seeding from an external site saying something along the lines of "are you sure you want to send data to MB from [xyz site]?", the Referer/Referrer header is used to indicate the sending site. That said, these days many common referrers like Harmony don't send values for this header, so MBS currently fills in the part meant to indicate the sending site with "null". Ideally, we should handle this a bit more gracefully.

Previously discussed in the MeB room on Matrix around this message, and further discussed with kellnerd as Harmony developer here.