I'd say a simple compliment like, "you look nice today" or "hey beautiful!" with a smile is fine.. but leaving a note on the car, is stalker speak for "I know what you drive"... and that translates further to "I can follow you home now if I wanted to".
No notes... and no stalking! (JK, but not really) **edit** NO FUCKING STALKING. When I posted this I had recently found a note on my car. This was a "tend and befriend" response.. I'm not friendly anymore. Fuck that shit, and FUCK whoever it was who left that note on my windshield. All these years later it's scary as fuck.
Keep it simple, and sincere.