The disk is found in the Presidential metro. Before boarding the train to the Adams Air Force Base, go up the opposite staircase toward the the Capitol building and you will find it next to a skeleton on a bench. It will start the unmarked quest The Sorrowful Suitor for receiving the all-nighter nightwear.
If someone finds this... please get this to my lover at the La Maison Beauregard Hotel in East Georgetown... he'll want to hear what I have to say. My darling... they've found me. I... I tried to get away... tried to get away so we could be together once again... I know you risked your life to get it to me.... combing the ruins, avoiding the Super Mutants... all for me. It seems I shall never lay my eyes upon your gift... you... you'll have to keep it and remember me every time you see it. I'm so sorry my darling... so... so... sorry I've let you down. So many have died for this thing... so many hearts have been broken. Please remember, I'll always love you... you will be with me forever in my spirit... I... I...l-love... you...
PC In some cases, listening to the holodisk can cause the game to crash, which makes it impossible to complete the unmarked quest. If you don't listen to the holodisk, Lag-Bolt won't appear at the hotel, meaning it's almost impossible to obtain the all-nighter nightwear. [verified]