I'm chiming in as I had the same issue/symptoms on my 2007 Edge about a month ago (57,000km).
When driving the car highway or on the move everything was fine. But I noticed when city driving (especially in stop and go traffic) this burning smell would come into the cabin. I googled the symptoms and sure enough I received a ton of returns on this 'PTU leak issue'. I booked my car into my local mechanic and sure enough he confirmed the PTU was leaking. Although not leaking enough to leave any evidence of fluid on the driveway. Basically the fluid drips directly onto your exhaust, burns, and voila smell filters into the cabin.
My mechanic didn't charge anything to get it up on the hoist and look at it. He recommended I take it back to Ford and see if they would cover it under power train warranty. The same day I dropped the car off at Weiland Ford in Kitchener, ON. Once the issue was confirmed by the ford tech the repair was covered 100% under the power train warranty.
Its been a month and no sign of further issues - although I have read on other forums this issue is known to re-occur in some cases. I'm hoping I don't have this stroke of luck.
Hope that helps.
..Liam