Strong sewage smell intermittently present for 6 months or longer. Frequently smell outside, particularly when air is thick on humid days. Occasionally smell inside the house. Appears to come from downstairs powder room bath, perhaps from beneath the sink. Have checked trap outside house; nothing. Neighbors smell the same thing, has been bad inside their house. They have had multiple plumbers out, focusing on the vents (did not know we had problem too). I have walked our lot and cannot find any apparent leak. Outdoor odor appears to originate from the direction between our two houses but cannot say for sure. Any ideas? Thanks!