From the halls of Blizzard comes a change to the way the Blizzard Authenticator functions, as some of you may have been wondering why you may not have been prompted for your code:
Blizzard, in their ongoing effort to improve player ease of use, has decided to tweak the authenticator system so that players do not have to give their code each and every time they log in. This is predicated upon whether players tend to log in from the same location more often than not; if your login location is consistent, the prompt may not occur.
Here's the official post:
If you use an authenticator – and we hope you do – you may soon notice that an authenticator prompt may not appear with every login. We’ve recently updated our authentication system to intelligently track your login locations, and if you’re logging in consistently from the same place, you may not be asked for an authenticator code. This change is being made to make the authenticator process less intrusive when we’re sure the person logging in to your account is you.