VALLEJO, Calif. — On a suburban block with six family homes, palm trees and views of the surrounding green hills, nothing at 110 Windsor Court stood out. Its occupants, who had moved into the foreclosed house a few years earlier, were quiet types.

Until the noise from falling roof tiles alerted neighbors to a fire there one recent morning, and Stephen Snowden, who lived nearby, banged on the front door. Nobody was inside, but firefighters discovered that the house had been converted into a type of illegal business found increasingly in suburbia: a marijuana grow house.

The entire second floor of the five-bedroom, 2,251-square-foot home, as well as parts of the first floor, was used to cultivate marijuana plants.

“They just blended right in,” Mr. Snowden said of the residents. “They left early for work and came back late in the afternoon. They mowed their lawn, took out their trash and got groceries. There was never any extra foot traffic.”