>That was likely someone not authorized to work in the US writing random digits on an I9 form.

I used to work a job years ago with lots of people who snuck in here. In order to get the job they needed to provide a social. Not having any idea wtf a social security number was, just that they needed one, it was a relief when someone they lived with or met on the street informed them that xyz at location abc will sell you one for $100.

That's one spot where the identity theft rubber meets the road. And practically everyone's social has been leaked by now.