This location appears to be a pre-War plumbing store with four entrances located south-east of Mass Gravel & Sand in Cambridge. It is abandoned except for two radroaches that are in the storeroom part and some armed bottlecap mines. There is a safe behind the counter with a bottlecap mine in front of it. A large toolbox is up the metal stairs in the storeroom and across the wire bridge.
However, the "secret" is to be found on the ceiling. Follow the line of plungers and you will see some items tacked to the ceiling. These appear to be out of reach, but you can shoot the boards, and the treasure will fall to the floor below. The treasure is a syringer rifle and a caps stash.
Plumber's Secret only appears in Fallout 4.