Spam Users Registering for Unknown Website … How?

Hey all, I have a small Multisite setup of which non of the sites have a “register” page.

Somewhere, somehow, I’m getting spam registrations … see image.

How are users registering and is there a way to find out where they’re registering from?

See image of what I mean: image link

Thanks!

The page I need help with: [log in to see the link]