After about 20-30 minutes of driving, I can smell something hot/burning coming from the underside. It's unpleasant. Initially, I thought it was the new brake pads settling in (dealer put this in, another long story), but the smell has been there for about a month now. It smells almost like when you drop some cooking oil on the stove or from a dirty oven that's been pre-heated to high temperature. My friend said may be the undercoat was getting heated too hot from the exhaust, but I did not apply any protective coats. Any ideas what could be causing this ?
I've been too lazy to take it to the dealer yet, but probably will be forced to spend 2-3 hours to take it to the dealer to check it out. Bummer.
Following Michael's advice, I checked the underside with a flash light and I think I found the culprit. I saw a dark spot about the size of silver dollar on the exhaust pipe under the rear passenger seat. It was hardened when I touched it, but it showed signs of having been melted. It did not alike an asphalt, but some other resin.
Now the problem is to find a strong solvent to remove it from the pipe... maybe I'll try nail polish..