Last couple of days I've detected a slight smell of petrol after shutting off the engine on the V70. My boy's car seat is on the rear OS so I park up, switch off and in those 30 seconds it takes me to get out and open his door and get his seatbelt off, I can smell the fumes. I haven't smelled it at all with the engine running (and my wife has crazy super acute sense of smell so she would have noticed any fumes inside the car weeks before I did!). I tested it yesterday, with the engine running, I had the car seat out and the seat base folder up, pulled back the foam bit under the seat and I was sniffing away in that area and down by the wheel/sill and couldn't detect anything until the engine was shut off, then had fumes for about 30 seconds.

Questions:
1) Is there any logical reason why fumes would only be forced out (or even an actual petrol leak) in the 30 seconds after shutting off the engine?
2) What should I be checking?

I've done the obvious and had a quick look under the car at the filter and surrounding area and no sign of a leak (filter is only a few years old too). Tank was filled up a week ago, and not overfilled (just to the first click on the pump, and it took the expected amount). Fuel consumption on the computer doesn't seem any worse so pretty sure there's not an actual significant leak.