I've always thought it was supposed to indicate that a/c was installed in the car. But like you, I've seen non-a/c cars with the sticker (including my own SL).
On one car I saw, the sticker was in the window, the car had no a/c, but it had a bracket for a compressor installed. Plus the original owner told me it never had a/c. It was as if the car was prepped for a/c, but it was never installed.
Douglas Kim
New York, NY
280 SL #016220