Someone could write an entire blog about smells. Here's one that comes up now and again when I'm showing houses. It came up for me the other day and had me stumped until I thought about it.
The smell is sewer gas. All drains in your home should have a trap. A trap is a curve in the drain that collects water. Gas from the sewer or your septic system is prevented from coming up through the drain into your house.
If you smell a bad smell in a strange place and you can't locate its source, think sewer gas. The smell might come and go and it could be worse on a windy day.
It may be coming from an uncapped pipe without a trap, but its usually from a drain that doesn't get much use. In time the water in the trap dries out and the gas finds its way into your home. Its an easy fix. Just pour a few cups of water down the suspected drain.
Worst offenders are basement drains. Do you have one covered by a rug? Having a water softener or furnace condensate line in a basement drain assures the trap never dries out.
Here's something related that has appeared several times on inspection reports for older homes. In an older home floor drains sometimes have a clean out plug in the throat of the drain. The clean out hole bypasses the trap so that a plumber can run a plumbing snake down it.
If the plug has been removed or lost sewer gas can come out of the drain even if there is water in the trap.
Old drains rust and sometimes you can't get a replacement plug to fit. Your local hardware store will have a metal and rubber expansion plug, similar to the drain plug in a boat, that will do the trick.
I can think of other sources of phantom smells. I'll save that for another Post.