If you talk your way in, that doesn't count as a mission that it's possible to get Smooth Operator on. I talked my way into the police station on my first run and still got Foxiest of the Hounds. If you didn't get the achievement, you must have set off an alarm and not noticed. It's easy enough to do - there are certain areas in Highland Park, for instance, where you can't hear the alarm sound and so may not be aware that it's going off.
Originally Posted by MarlonShakes
Overall though, I really don't see why this achievement is confusing so many people.
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong."