So here's the deal. I am building a new house which is basically
done, sheetrocked, finish floors, etc. The only problem is that we
can't get a plumbing final passed--seems like there is a fitting that
has come apart in the vent portion of the DWV. I am looking for some
brilliant ideas to find it, short of filling the system with water and
looking for the leak.. We have tried the "peppermint test" --dumping
peppermint down the vent, follwed by hot water, and can't smell
peppermint anywhere. We also tried a camera, but were unable to fish
it in the lines smaller than 3".

A few ideas I've had are--

1) devising a hillbilly smoke test--setting a shop vac to blow. Put a
little water in the tank, and add dry ice, and hook this up to pump
"smoke" into the system.

2) Hiring a guy with an infrared camera. Attach a heat gun to the
system. Pump heat and look for heat.

Anyone have any other ideas?