Exhaust smell

First i have read quite a few posts on this subject on here and some other sites. I'm getting some exhaust smell in my cab (Not diesel but exhaust)
Now I crawled under and over the top with my Mirror and could not see any soot on my Y-Pipe anywhere.
Are there other places that people recommend to look for soot? If not soot can it still leak in places?
How long does the smell last inside? I'm asking this as i have had some real cold white smoke starts. (I had it in the body shop for a few weeks and when i went to pick it up they had just started it and the truck was in a cloud of white)
I'm thinking my batteries may not be supplying enough juice to heat the glows up properly or a glow is going. I recharged them after the shop and it started with less smoke. If i plug in there is no smoke.
So I am wondering if the cab gets smoked out a few times (in the body shop visit) would the smell get in the fabric? I febreezed and waiting for some sun to air it out for a day.
Sometimes driving i get a whiff, sometimes not. However i was parked for a few hours and can still smell it in the cab

Thanks and cheers
Hugh