How is this an "epic fail"?
When you start your computer (assuming you are using your Microsoft Account) the screen will offer you a choice of sign in options (accounts) and you sign in with your own account info and password/pin/whatever.
Yes, that will sign out anyone else who may have been logged on previously and they would (possibly) lose unsaved work. But, unless they are unbelievably thick they would have saved their work before shutting down the computer anyway so, really, any such risk is negligible.
The same is true for local accounts. Just go ahead and sign in with YOUR password and account info.
And when you're ready to sign out say "Hey, Cortana, sign out."
You can also tell her to shut your device off or lock it if you're going to be away for a bit.