Just after installing Windows 10 Pro, I boot up to see a BIOS looking screen for a half second before the Windows 10 loading screen. It's only there for a split second before booting, but is there longer when restarting rather than a cold boot. I have a custom built system with UEFI, so I'm not sure what's going on.
Sounds like a glimpse of the EFI shell. Nothing to worry about... until you cant get past it...
Consider it the built in command prompt for the bios. and the EFI bios is no longer just configuration data.
You can google for EFI Shell commands if you are interested in what can be done.