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Park Street station, down the stairs, straight across all the tracks and in a Novice locked room on the left after reaching the far wall. National Guard training yard, upstairs in the smokey room.
Hester's Consumer Robotics, in a case on the racking in the entrance room. The key is found in a chest a few rooms down. Hardware Town, upstairs behind the locked door. Up to three can be found on Kellogg after defeating him in Fort Hagen Command Center. The Stealth Boy is by the two skeletons in the room on the other side of the gap. Go down the stairs to the metal walkway, then jump across the gap. After entering, turn right at the first intersection of hallways (with the construction light, just past the two can traps). Eager Ernie is up in this area, the Stealth Boy is inside an open compartment on a machine to the left of the Master control terminal. One can be found upstairs in the main house on the track. The other is downstairs in a locker located in the hallway with large mounds of rubble on the floor. Two can be found, one tucked between two lockers near a skeleton that is draped over the top of a ladder. The armory is accessible both during and after the Old Guns quest. Two can be found in the armory, behind the large metal door. Boston mayoral shelter, next to a dead soldier. Boston Public Library, in the locked storage room. It can only be obtained if one sides with Bobbi. Three can be found at the end of The Big Dig inside Hancock's stockpile. BADTFL regional office, in the evidence locker.
After picking the locked door in the hallway, it will be on the shelving to the right.
Back Street Apparel: After entering the building, enter the first door on the left. Found in an Advanced locked safe in the dark storage room behind the counter. In Back Alley Bowling, part of the General Atomics Galleria. Once through the door, check behind the collapsed floor/ramp to the right to find the Stealth Boy sitting on a metal shelf. ArcJet Systems, through a Novice locked door to the right of a collapsed stairway in the small lobby with a broken, flooded floor. They can be found scattered all over the Commonwealth or they can be sold by merchants. Instead, it is activated by hand and is worn on the waist. Unlike other variants of the Stealth Boy, this version isn't worn on the wrist. The Stealth Boy is a portable device that, when activated, renders the user invisible for 30 seconds.