Is it quite an old car? Older cars don't have such good chokes as newer models and quite often smell of petrol when they're cold.
Also - this may sound silly but the wind direction has been different for the last day or two - it could be blowing the fumes back to you when the prevailing wind carries them away. Just a thought.
Have a good sniff under the bonnet and run your fingers under any pipe unions you can see, while checking them for wetness and smell. Your nose is your best detective here!