Engine fumes entering cab on 2007 S40

2007 Volvo S40 with 15,000 miles has smell of engine odor in the cab when window is opened an inch or so. The engine was detailed 4000 miles ago and part of the smell is the fumes from the caustic soap that was used. My question is: How is it getting into the cab. Do the fumes come out the back of the hood and then get sucked into the vent or heater inlet? There is no smell when the window is left closed. Thanks for any help, 831johnc