Dirty under the hood!
At 15,000 miles/20 months, it's pretty dirty and dusty under the hood. I did an oil change and really needed to clean off the area around the oil cap. It's like an old attic under there. I've had several new cars of various makers over the past dozen years - and this is certainly the dirtiest under the hood. Not sure if the rubber seal at the front of the hood is doing it's job. Anyone else?
Former driver of a 2011 SLS AWD Platinum
Don't think the bonnet seal would have anything to do with it. The area under the bonnet isn't "sealed"- dirt laden air can come through the radiator core, up from underneath. The dirt has probably got more to do with the areas you've driven and the weather. 

David
If you look into the grille and around it etc theres a fair bit of moulding etc to direct air flow into both the intake and the engine bay, more minor details than most cars ive seen, im guessing this will also push in a bit more dust etc. When i take my car on a dirt road i regularly use its always hugely dusty in there compared to previous cars
2011 Premium Silver XL
Learn. Do. Break. Fix. Explore. Save. Repeat
Learn. Do. Break. Fix. Explore. Save. Repeat