This location appears to be a pre-War plumbing store with four entrances located southeast 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. Following the line of plungers, one will see some items tacked to the ceiling. These appear to be out of reach, but the boards can be shot and the loot will fall to the floor below.
- Short syringer rifle - on the ceiling where the trail of plungers ends, with a caps stash.
- Vault-Tec lunchbox - behind the counter with the cash register.
- Four bottlecap mines - one to the right of the red forklift, one on the shelving across from the checkout counter, one behind the counter and one directly up the stairs.
Plumber's Secret appears only in Fallout 4.