The moral of the story? A sewage smell is almost always a plumbing problem—not a paranormal one. Start with the P-trap. Then check the toilet ring. Then the roof vent. Work your way up the chain of stink.
If nothing works, don’t suffer in silence. Call a plumber. Because living with that smell is like owning a pet skunk. Cute? No. Tolerable? Absolutely not. Now go pour some water down that guest sink. Your nostrils deserve a break.