Come to think of it, the Volvo S60 I traded in for the IS250 also had a strange A/C smell on start-up. We don't have a whole lot of rain or humidity in Melbourne, despite what the Sydneysiders would have you believe, so I doubt that was the cause of my car's smell either.
I always thought is was a dead animal or a rotting rubber seal. As the car was 5 years old and being traded in, I never persued it.
In your case, I would get the dealer to look at it. Although if it was anything like the smell I had, it was gone after the first minute of air blowing through and may be hard to trace.
Good luck with it and keep using the Rexona