This note could be from the Gunners, located at Gunners plaza, from the raiders at Quincy Quarries, led by Slough, or from the raiders inhabiting Hyde Park themselves, to the previous residents, due to the number of settler corpses strewn around the buildings.
A terminal entry in Gunners plaza suggests that the Gunners have infiltrated the Hyde Park raiders with the intent of destabilizing and undermining them, either to take over the area, or to simply reduce their threat. They mention the raiders have become aware they are infiltrated, but have not identified the double agent. The entry suggests that they can exploit this "witch hunt" by spreading further suspicion and paranoia. A knife in the leader's pillow with a threatening, anonymous letter, would fit with that plan.
Looking forward to seeing you.