OK, it was quite easy. I inspected things visually and by hand after the car was parked overnight. I identified two possible problem areas. Then I turned on the engine and felt for blowing starting from closest to the engine and working back.
The two areas I suspected at each end of what I think is the pre-cat were blowing a bit, and each end of the muffler where it's mounted to the pipe seemed to have a bit of air escaping.
^^^I'm not sure how much having that done at a shop would cost.^^^
The pre-engine-on inspection was crucial, as there's only about two minutes before the whole pipe system is scorching hot.
For someone who treated his first car, a CR-V, like it was a taxi, I'm getting pretty good at this mechanical stuff, and it's only been a month with my new baby. I guess it shows the love I have for my Runner, because I really do give a **** about every minor thing. I guess having CO in the cabin isn't minor, though.