Although different from a sewage odor, maybe you have rain getting into your wall cavity. Wet, moldy insulation and drywall is smelly stuff. If your smell returns a few days after rain, that could account for it. The fact that you smell something at the wall plate is a clue.

Thanks for the reply but there isn't any way water could get into that wall. It's a sewage smell. It went away yesterday.

You should read my post "has anyone used this".

You should get a smoke test. They set off a smoke bomb, made especialy for this, and they blow smoke into the plumbing and venting. Then you look to see where the smoke comes out. I wish I had done this years ago.

The smell IS BACK this morning.

freedee, I read your other post and have been meaning to reply to it. Too many outside things to do before the snow starts. I'll get back when I have a few extra minutes to think.
thanks for answering.

Get a roll of painter's plastic and blue tape. Isolate each room. Wait several hours, then smell behind each sheet. Repeat as necessary until you narrow down the location.

As for that wall switch you mentioned in the original post, is it on an outside wall, or on a wall under a staggered roof line?

The smoke test (or maybe the peppermint test) is the right suggestion. You may have a sewer vent pipe running through that wall that has a leak due to a nail or disconnection. This leak is allowing sewer gases into the wall cavity which are leaching out into your room via the electric switch/outlet.
This problem may be more apparent in cold weather due to gases sinking as they get colder.

How about an update? Problem solved?

The smell has been gone about three weeks so we figured the smoke test wouldn't work. There is no sewer vent pipe in that wall.

This has happened every once in a while for years.

Thanks for asking homebound.

This happened at our weekend lake place one winter. The smell was in an upstairs guest bedroom. The origin of the problem was the drain in our laundry room which was a level below. The water in the drain had evaporated from lack of use and septic odors were coming in. We've prevented it since then by just making sure we run water down there occasionally.

If there has not been any remodeling or plumbing repairs, a broken pipe or nail hole would not be my first suspect. I would check for a clogged vent pipe first. get on the roof and run a garden hose down each plumbing vent. Don't turn the water on until you are sure you can get to slab level. Squirrels, birds and just tree droppings will sometimes plug the vents.

About there being no sewer vent pipe in that wall. When our smoke test revealed where the opeing was, it was about 20 feet from where we smelled the odor the most. it was behind a wall and was enclosed is that wall. It drifted pretty far from it's sourse because it had a straight opening behind a wall. It would sometimes go elsewhere if the air was moving, like when the fan in the powder room pulled the air up.