Steam had an update and I had to log in again. Once I logged in, a window popped up asking for an email verification code. The header on the window said "Steam Guard." I don't have steam guard enabled on my account. Why is it asking for a verification?
According to a Steam Guard post:
Steam Guard is enabled by default on your Steam account if your email is verified and you have restarted Steam twice on your computer since verifying your email.
It sounds like you've been "auto-enrolled" into this form of Steam Guard.