It’s means he’s a fan who is also an actor, meaning he’s likely to be at the Sony Studios lot for any number of reasons (which is where the photo was taken.) The fact that rumours linked him (and were later shot down) to a third Ghostbusters is intriguing, but that’s it. Fact is, simplest answer is that a long time fan got to see the car we all love, and twittered it – like any one of us would in the same situation. No biggie.
Thanks to Grim for the heads up!