We recently started smelling a sewage smell in our shower from what we thought was the drain. Our home is only 4 years old, and we don't think it should be having major plumbing problems yet, but... We've tried the baking soda and vinegar trick, bleach, checking outside vents to make sure they are not clogged, etc. Tonight we pulled the strainer off the drain, put a hanger down it, but it was not clogged. Then we pulled off the "collar" (not sure I'm using correct name) that was at the top of the pipe under the strainer. That area was full of gray gunk that is the source of the stench. Wondering if there should have been some sort of seal put on or why this gunk is built up under the shower floor? What can we do?