Let's save the question while I explain what your wife was in no shape to hear. A moving car creates a low-pressure area behind it, a sort of suction drag. A car does not run away form its own exhaust. It pulls some along with it. Station wagons are especially prone to do this.
Some people leave the rear window of a wagon open for ventilation.
That sucks in exhaust. If there'd been no hole, they'd have seeped through the seat back, but more slowly. As for your question, I checked the exhaust system on the spot. Fennel swallowed in obvious embarrassment. Instead of asking dopey questions, I should be thanking you over and over.