I notice a slight gas stain on the driveway under the engine compartment of my Volvo XC-90 when it rains

There seems to be a pinhole gas leak somewhere in the engine
compartment of my 2004 Volvo XC-90.  When it rains, I notice that there is
sometimes a rain puddle on the driveway that has gasoline in it or else a
gas stain (rainbow colours) on a drier portion of the driveway.  Typically,
this occurs right after I back-up the vehicle a few feet towards the street.  
The location of the leak appears to be from under the engine compartment.  
I took the vehicle to a qualified mechanic (Canadian Tire) and had the fuel
lines and components inspected - the mechanic indicated the fuel lines and
components were fine.  Also, the check engine light has not come on,
meaning the gas vapour sensor has not detected a leak in the fuel system.  
Nevertheless, I see evidence of a leak when it rains and immediately when I
move the vehicle (ie, backing up).

My theory is that rainwater collects somewhere in the engine compartment
near a pin hole leak that is too small to detect.  The leaking gas combines
with the rainwater over some hours.  Then, when the vehicle is moved, the
rainwater mixed with gas is disturbed and flows off the vehicle (I have
watched dripping for a minute or so after moving the vehicle a few feet).  
This then leads to the gas stain.

Questions, is this dangerous?  Comments from anyone?  

Thanks,

Mike