The location of the Seward Square entrance is across the street from the Seward Sq. North Metro in an alley that spawns a combination of 3 super mutants and/or cantaurs. Two meet you going to the Sewer grate and one spawns behind you as you reach the grate.
The Pennsylvania Avenue entrance is very close to the Metro Central map marker. From there, head north around the other side of the station to the diner. Go west through the opening in the ornate wall and veer northwest around the corner. As soon as you get past the rubble and reach the paved street, look to the right (north) for a grate on the sidewalk next to the rubble.
The Sewer interior is a medium-large size metro tunnel, with 3-4 super mutants and a high possibility of spawning multiple super mutant overlords if one has Broken Steel. A door inside the tunnel opens up on a room, located in the middle of the local map, with stairs that leads to a small upstairs area with several skeletons and a holotape.
The stash, mentioned in the hidden stash note, is in a dead end in the southeast area of the metro. It consists of several ammunition boxes that are hidden below a large piece of plywood in a blown out portion of the left wall.