Is there any chance that there is a duct booster fan in the duct to the vent where you wife saw the smoke? A dying blower motor would produce some smoke and stench for a short while as it burned the insulation off of its windings...
Just a thought!
Unexplained smoke is usually electrical. But very rare other things... How is your house heated? Also, how is the sewer venting pipe in your house? You dont have these do you »www.oateyespanol.com/sure-vent/r···_app.jpg ?
