When the weather gets really cold (below 30) I intermittently smell a gas/exhaust smell in the cabin but also on the exterior. I have on two occasions had my mechanic check this and each time he has replaced an “old and cracked” hose. He has to use the smoke tracer to find where it is leaking which adds about $300 in diagnostics and labor to replace a $20 hose. On both occasions he urged me to make the repair immediately as it could be extremely dangerous. (Makes sense if I am smelling gas.) After dropping over $300 a month ago for the second hose replacement, we have had another cold spell and I am smelling it again. I am wondering if this could be an exhaust/heating system problem. The car is 11 years old with 120K miles and I want to keep it running a few more years. I am due for front brakes and new tires soon but do not want to invest that $$ if this gas thing is going to keep costing me $300/occurance or a new exhaust system. Any ideas or suggestions?
Has the mechanic been able to get a whiff of this smell himself? Or is he just operating on the basis of your reports? He has to be testing the evaporative emissions system for leaks as it’s all I can think of that would make sense. But obviously that might not be your issue since the “fixes” don’t seem to be “fixing” anything.
Describe better what pattern there is - if any - to when you smell it. Is it mostly earlier in the day or your drive when the car has been started from cold? Or just throughout the day any old time? Is it, perhaps, only while you’re sitting at idle?
Note that when the weather is cold you car runs “rich” (lots of gas) until it warms up. The exhaust can be quite stinky and its quite easy for it to follow you into the car when you get in and out. This is especially so if you open up the back hatch.
Gasoline fumes and exhaust smell quite different. Can you think a little more about which it probably is? You frequently get a really clear idea of the smell of gas fumes everytime you get gas.
Next time it’s cold open the hood and see if the smell is stronger there. It could be your purge valve on you charcoal canister is bad or a leaky fuel injector o-ring.
I have the exact same problem in my 2016 Subaru Forester (purchased new September 2015). Service manager recently told me that all cars smell like this. 
I have tried to get them to resolve this issue, but they can’t seem to find the problem.