Cold Case

Cold Case is a side quest in Fallout 76.

Detailed walkthrough
The Park Security System is inside the ruins of a building. Access the machine and check the Holotape Review - the text talks about a missing kid. October 23 mentions a tape. Reading this will update the mission. The next step is to locate and play two holotape clues. One is nearby, the other is a bit harder to find but the entrance to it is in the next building to the north. Go inside the fence at the back of the building and look for a staircase down. The tape is in the basement on a bookshelf.

Look inside the mailbox outside of the same building as the second tape, through the fence on the right. The Letter to the Media will be inside and pop up on the screen when looted. The note mentions a key. The next waypoint is at the top of a waterslide tower on a box. This leads to a KidSecure ID key that was made for Freddy Wood.

Go to Clarksburg and locate the Post Office. The mission will update when the player is close. Use the terminal inside to research the key. Search by Box number and read the entry for thirteen. The next step in in Otis Pike's house, some distance to the northeast. Inside the house is another terminal. Use it and read Correspondence/Dam Safety Inquiry. Gain access to the Grafton Dam building to complete the next step. Both doors are locked, but there is a hole in the wall.

After reaching the Woods Estate, go upstairs to find a desk with a terminal. Loot the note in the desk to progress the mission. The terminal has related information, but accessing it is not necessary to complete this checkpoint. Fast travel or walk back to Wavy Willard's Water Park and interact with Miss Anne to complete the quest.

Rewards

 * Stimpak: diluted
 * .308 round x12
 * Plan: 10mm SMG
 * Plan: Light metal legs
 * Bottlecap x95
 * Perfect Storm