A note from Little Moonbeam's father

 is a note Sydney's father left her when he was dying in the Statesman Hotel. If you give it to her after she tells you her story, she will give you Sydney's 10mm "Ultra" SMG.

Location
It can be found on the third and uppermost floor of Statesman Hotel Mid Level, close to the center. Just south of the marker for the Statesman Hotel Restaurant elevator (but behind a wall, so not directly accessible).

If you are making your way up to Reilly's Rangers for the first time, it is two floors above Theo. The room is the first door to the left (facing north looking down the corridor immediately after leaving the stairwell to that upper floor). The door is directly opposite a locked door. The locked door is a storage closet (easy difficulty) with ammo and med supplies. Just beyond the storage closet is a side-by-side fire hose and med station on the wall. Once you enter the hotel room, just look for the illuminated bedroom. The note is on a bed, next to a skeleton, under the ceiling light.

Transcription
Весточка от отца Лунного Лучика