The note is located in the Enclave research facility, on the corpse of a female Enclave scout (the same corpse that possesses the holding cell keycard). The corpse is on the floor of the room where the deathclaw fight takes place during Over and Out, outside of a holding cell, across from the central table with the radscorpion on it.
I won't be returning. I've already arranged things with Williams. He should have already reported to the Colonel that I had an "accident" on our last run.
These people treat us like shit and for what? We risk everything to bring back these grotesque creatures for them to poke and prod.
How many have we lost already? I can't lose you too. You know this world isn't entirely the hellscape we make it out to be. If you change your mind, you know where I'll be.