The most common smells we notice while driving with the top down emanate from the surrounding environment as we pass by – someone’s BBQ, a restaurant’s kitchen grill / deep fryer exhaust vent, farm animal / fertilizer, smoke stacks, etc., but there can be some smells from within the car itself that can serve as clues to potential issues.
Antifreeze coolant has a unique “sweet” smell, and those that have had to replace their water pumps are probably quite familiar with it, but be aware that the coolant overflow tank vents into the engine compartment when the engine is running hot, so an occasional whiff of that smell doesn’t necessarily mean you have a coolant system leak. Unlike most cars that have no aerodynamic belly pans, the overflow doesn’t just exit onto the ground, but is captured somewhat in that pan.
I see many questions regarding a “moldy” smell, and that is usually attributed to shutting off the car with the HVAC system in recirculation mode – Max A/C, Recirc, etc, which results in all of the vent flaps being closed, and yes, the trapped air is getting moldy.
Uric acid / Ammonia type smell
If you instead smell a uric acid / ammonia type smell, chances are good that a critter has hibernated in your engine compartment somewhere – most likely in the HVAC air inlet (underneath the cowl panel). See my article for more on this.
A new smell was mentioned in a forum question recently – a “Band-Aid” smell, which is attributed to leaking refrigerant oil, which is needed by the compressor, and circulates with the Freon™ 134a (the actual refrigerant) when the A/C is running in our 124 Spiders. Usually the leak occurs in the evaporator (requiring replacement), and is identified as a “A/C Freon leak” noted to be strongest when starting the car. Closely related would be a “garlic” smell coming from other components such as the condenser coil, since R134a itself smells like garlic (although the oil smell usually dominates).