Brotherhood note is a paper note in the Fallout 76 update Steel Dawn.
The note can be found in the makeshift vault, nailed to the pool table with a throwing knife next to the remains of a robot. It appears only after completing the quest Property Rights.
Is this distasteful barbarity the Raiders' idea of entertainment?
Make sure it's cleaned up. This is our outpost now, and leaving something like this around doesn't reflect well on the Brotherhood.