To solve this Murder Mystery, you'll first need to use Michael to find four cryptic clues scratched into the walls around Los Santos. These can be found at the following locations:
West Vinewood: The Gentry Manor Hotel
To the east of the main Gentry Manor Hotel building, look for an inscription next an archway that reads "OLD FRED ACT NICE, BUT IT AIN'T SO!"
Downtown Vinewood: Oriental Theatre
To the west side of the Oriental Theatre entrance behind some palm trees, you'll find the message "AIN'T NO MYSTERY, THEY DROWNED HER BECAUSE OF WHAT SHE KNEW!"
Alta: City Hall
In an alcove around the back of City Hall, the writing on the wall says "FRED MAKES CARTOONS FOR KIDS, FUNNY THING CONSIDERING WHAT HE HID."
Vinewood Hills: Galileo Observatory
On the lower level at the south side of the Galileo Observatory, you'll find this message that simply reads "DAVID RICHARDS KNOWS."
With the four clues gathered the next step is to locate the missing body, which can be found off the coast to the west near Fort Zancudo. It's best to take a submarine to this location (the Sonar Collections Dock is nearby) then exit it and use your scuba gear to swim up to the body.
The final piece of the puzzle can be found on the desk of Solomon Richards. To access it, you need to have completed the Meltdown mission then head to the marker outside his office at Richards Majestic after 21:00. You can then read the letter on his desk, revealing a dark secret hidden in a mine up in the hills.
Out in Great Chaparral you'll find an abandoned mine, where you can use explosives to blow the doors off and gain access. Follow the rail tracks into the mine and you'll eventually discover a body in a pinstripe suit, which should unlock the option for new Vintage filters.
Vintage Noir Filters
With the Murder Mystery solved, you can now modify your display with two new Vintage Noir filters. These can be accessed by visiting the body in the mine, or by calling the new Isaac contact on your phone to cycle through them.