Admin Login Pop-Up

Greetings all,

Has anyone else seen a plain blue modal window pop up on reboot of Win10 Home edition asking for the admin user password? This is relatively recent, and I've not seen it previously until several weeks ago.

I say modal, but if you look closely you can find a X to close it. However, this is concerning as there is nothing to indicate that it's genuine or otherwise an official part of the OS. Unless it's changed along with some recent update, it hasn't been a part of the normal boot-up routine until now. 

Thanks in advance