Example: the basement smell that showed up the week after closing

The situation

Illustrative example. Not a real job or customer.

First-time buyers close on a 1970s bi-level near the foothills side of the metro. At showings the lower level smelled faintly of fresh paint and a plug-in air freshener. The house had been vacant for about three months before closing. During their first week, with boxes stacked downstairs, the lower level smells sour near the laundry room and musty in the bedroom on the downhill side.

1. Before anything else, they confirm the smell is not near the gas furnace or water heater and is not a rotten-egg gas odor.
2. They find a floor drain under a rug in the laundry room and pour water into it, then run the unused basement shower for a minute.
3. By the next day the sour smell near the laundry is gone; the musty smell in the downhill bedroom remains.
4. In that bedroom they pull the bed away from the exterior wall and find fresh paint over a faint water line and slight swelling in the baseboard.
5. Outside, the window well on that side is full of leaves and silt, with no visible drain.
6. They gather their disclosure documents and inspection report and send a polite written question to the sellers through their agent asking about past water in that room.
7. They clean out the window well, add a cover, extend the nearby downspout, and run a dehumidifier while they wait for answers and monitor the wall.

The sellers' agent responds that the window well had overflowed once in a heavy storm. With the well cleaned and covered and the baseboard area opened and dried, the musty smell fades over several weeks. The buyers keep the correspondence with their closing papers.