Francis McConnell is a field supervisor for the Philadelphia Water Department, but lately he is acting more like an undercover police officer.

Several hours a day, five days a week, he stakes out junkyards. Pretending to read a newspaper, Mr. McConnell sits near the entrances and writes down descriptions of passing pickup trucks and shirtless men pushing shopping carts.

His mission is to figure out who is stealing the city’s manhole covers and its storm drain and street grates, increasingly valuable commodities on the scrap market. More than 2,500 covers and grates have disappeared in the past year, up from an annual average of about 100.