Thanks.

I called my gas company. The technican came used a digital gas meter to
measure the amount of gas at the vent openning. The reading fluctuated
between 0 and 0.05. I don't know what the unit is. The technican said
"it's practically nothing" and don't worry.
As I smell now, I can only smell gas when I put my nose next to the
vent. But if I move away a few inches, I don't smell. (based on what
Stretch said this is normal)

Maybe what my kid smelt (which was a little further) was because of the
load changes.

Stretch's "NOTE" makes me feel much better. But some of the responses
said that there shouldn't be ANY gas out of that vent. So I am still a
bit concerned.

Do you guys think it's okay to trust what the technican said?