35iS Antifreeze smell

I picked up my 35iS 2 days ago and it goes really, really well - but I have one concern in that when I get out or am sitting at traffic lights I sometimes am getting a waft of coolant smell.



This is with the roof down and definitely from my car as it has happened maybe 6 times. It is strong enough that a friend mentioned it.



There is no smell in the car when I first get in it and the floor is not damp, nor the car smelly. I can get the smell through the air vents a bit when the air con is first turned on but not after that via the vents - as it has been on with the weather so hot but the general smell is from outside with the roof down.



I open the bonnet and can't smell or see anything. The level seems ok but the cap was half a turn off tight and it doesn't click like the manual says to confirm shut (but clearly won't go tighter more than I have now turned it + the arrows line up now between reservoir and cap).



Any ideas ? I may ring the dealer (though they are miles away) or take it to a local BMW dealer just to check for peace of mind. There are no puddles of coolant .... just the smell.



The car is a 2011 with 28K on it.



Any comments appreciated.