Well, talking to absent dogs, to be more precise -- but invisible sounded catchier.
When I'm home alone with The Dog, and she's awake, and we're not in the middle of tug-of-war or some such, I tend to think out loud at her. Whether it's more to make her feel like she has company, or to make me feel that I do, I won't try to assess.
So now, when I'm on my own in Kroger or someplace, I find myself thinking out loud. About nothing more interesting than milk or sandwich bags. I have probably been on the verge of talking to myself in public places anyhow, but The Dog has pushed me over the line.