2012 Lincoln MKZ PTU Leak
Had the same Problem with my 2012 AWD MKZ, 44,000 mi. We had a "Propane" type smell through the air conditioning vents and outside the vehicle. There is SSM 45053 which covers this issue. It looks like this is a common problem with quite a few Ford and Lincoln vehicles. When I first brought my car to my Lincoln / Ford dealer, they said that intermittent problems are sometimes hard to find. I told them about the SSM. They were able to find and repair the leak. See SSM 45053 below:
SSM 45053 2007-2015 Various Vehicles Equipped With All Wheel Drive Propane, Or Natural Gas Odor During Idle Or Low Speed Driving
Some 2013-2015 MKZ vehicles equipped with a 3.7L, 2007-2014 Edge, 2007-2015 MKX, 2011-2015 Explorer, 2009-2015 Flex, 2010-2015 MKT, 2008-2015 Taurus, 2008-2009 Taurus X, 2009-2015 MKS, 2007-2012 Fusion/MKZ, and 2007-2011 Milan vehicles equipped with a PTU may exhibit a propane, or natural gas odor from the HVAC system after driving the vehicle for 20-45 minutes. This odor, if present, is most noticeable while idling or during low speed driving. Inspect the PTU for fluid leaks and repair any leaks per Workshop Manual Section 308-07B. If odor is still present after the leak repair and cleaning, replace the PTU and vent hose, if equipped. For vehicles with no leaks and odor present, replace the PTU along with the PTU vent hose, if equipped. Refer to Section 7 of the SLTS manual for the appropriate labor operations.