It began with a smell — faint, sour, and fleeting — drifting through the hallway like a whisper. At first, Tom Fisher, a 42-year-old homeowner in the suburbs, didn’t think much of it. He assumed it was something simple: a forgotten onion, old food in the trash, or maybe a spill behind the stove. He scrubbed the kitchen spotless, aired out the house, and went to bed thinking he’d solved it.

By morning, the smell was back.

At first, it was subtle enough to ignore. Continues…