UNKNOWN EMAIL Sign in with your Microsoft account to continue app experiences across your devices

Hi all,

I turned on my pc this evening for the first time since updating it last night. Upon booting up, I got a notification called "Sign in with your Microsoft account to continue app experiences across your devices." I was going to do so, however, when I click sign in, it prompts me to set up Windows Hello with an email address I haven't heard of before. It does not give me an option to switch email. I am completely baffled by this, as I have no other users on my computer, and the Microsoft account I use for my computer is my familiar, everyday one.

Does anyone know how on earth this could've happened? And even better, how I can make sure only my email is associated with my pc?

Many thanks.