The easiest cause to fix is a dry P-trap. Every drain — sink, tub, floor drain, an unused guest bathroom — has a P-shaped trap that holds a small amount of water specifically to block sewer gas from coming up through the pipe. If a drain hasn't been used in a while, that water can evaporate, and once it does, there's nothing stopping sewer odor from rising straight into the room. This is common after a vacation, in a rarely used floor drain, or in a guest bathroom. Running water for a minute in the affected drain refills the trap and typically resolves the smell immediately.
If running water doesn't fix it, or the smell keeps returning, the cause is likely something that needs a professional. A cracked or disconnected vent stack — the pipe that runs up through the roof to let sewer gas escape outside instead of into the house — can leak odor into a wall cavity or attic. A failed wax ring under a toilet lets sewer gas seep out at the base of the toilet every time it's not perfectly sealed, and it's easy to miss because the toilet still flushes normally. And a break or crack in the sewer line itself, sometimes from tree root intrusion or ground shifting, can let odor (and worse) escape underground and work its way up.
Because several of these causes look and smell identical from inside the house, diagnosis matters more than guessing. A sewer camera inspection lets us see exactly what's happening inside the line without any destructive digging or demolition — we run a camera down the line and can pinpoint a break, a root intrusion, or a disconnect precisely. See our sewer line page for more on how camera diagnostics work.