The note is added to the player's Pip-Boy after reading the #Get Passcode entry that appears after answering with correct prime numbers on three Museum Information terminals in the Museum of Technology during Jiggs' Loot.
Nice job, Jiggs!
I knew you'd remember the good old days! The loot is in the Security Office Safe in the upper part of the West Wing. Use the terminal up there to get in. Enjoy your share, pal... you earned it! Meet me in the old diner outside the Jury Street Metro Station. See you there.