Smell from vent in wall?
This looks like an air admittance valve.  While not common, these can fail in an open state.  You could be smelling sewer gasses escaping from the stuck open valve.

When I had one of these stuck open, it was very obvious that sewer gasses were escaping, and I could temporary close the valve by "tapping" on it with the back of a screwdriver.  This was only a short term solution, and the valve was replaced to prevent it from happening again.