It can be found in The Brass Lantern in Megaton and permits access to the terminal in that location. The note is jammed in a corner of the terminal room, behind the chair closest to the corner. However, the chair and wall overlap the note, making it almost unreachable. The most accessible way to retrieve the note is to stand on the chair, crouch and look down in the corner. Aim at the joist or frame of the corner which covers the note.
The terminal access password is: "Lhats"
In the data section of the Pip-Boy its name is Brass Lantern password.
Behind the scenes
The password spelt backwards reads "Stahl".