If you just moved into a house, flush every drain you're not using regularly

Every drain in your house has a P trap underneath it. It's a curved section of pipe that holds a small amount of water. That water is the only thing blocking sewer gas from coming up through the pipe and into your living space. When a drain doesn't get used for a while, that water evaporates and now you've got an open path from the sewer line into your house.

Guest bathrooms are the worst for this. The sink, the tub, and the floor drain if there is one all have their own trap. If nobody uses that bathroom for a few months, every one of those traps can dry out. Same with basement floor drains, utility sinks, wet bar sinks, and laundry drains in rooms that don't get used often.

The smell is the first thing you notice. People describe it as musty, rotten, eggy, or just off. It comes and goes and it's hard to pin down because the gas drifts through the house and doesn't always concentrate near the dry drain. I've been in homes where the homeowner was convinced they had mold somewhere and spent money on testing. Turned out to be a dry P trap in a basement floor drain they didn't even know existed.

The fix is simple. Once a month walk through the house and run water in every drain for about 30 seconds. Every sink, every tub, every shower, every floor drain. That refills the traps and keeps the seal intact. If you have a drain you can't easily run water into, like a floor drain behind a storage shelf, pour a cup of mineral oil into it. The oil floats on top of the water in the trap and slows evaporation significantly. Lasts months instead of weeks.

If you just bought a house and it sat vacant for any amount of time before you moved in, do this on day one. Every drain. Run water through all of them. A house that sat empty for even a few weeks can have multiple dry traps and the combined sewer gas from all of them makes the whole place smell off. People blame it on the house being closed up and think airing it out will fix it. It won't because the gas keeps coming until the traps are refilled.

Takes about five minutes to hit every drain in a typical house. Beats spending a weekend trying to figure out where that smell is coming from.