I get a sewer smell in my lower bathroom. The room has a shower, toilet, and sink in it. The lower level is on a slab. The smell only happens when I have the bathroom window open and another window or door open on the lower level. I have covered the shower drain and disconnected the sink drain and plugged it with a rag and tape. I still get the smell. The toilet water does not move or bubble. There is no smell outside the window.

My wife thinks the smell is coming from the sink. The sink vent ties into other vents that run to a roof vent.

How do I determine the source of the smell and eliminate it?