With the Epsilon Program complete, Marnie will send you a clue about finding an Epsilon Tract - there are 10 total to find, and Marnie sends a new clue every time you find the previous one. You've already unlocked the achievement/trophy at this point, and the tracts are simply for collecting. There's no reward for finding them all. However, we know you guys will still want to track them down. So here you go:
Tract #1 - Mount Chiliad
The first Epsilon tract is atop Mount Chiliad. Head to the observation deck on the summit to find it.
Tract #2 - Palamino Highlands
Off the east coast of the Palamino Highlands is a bay. Take a dinghy out there and go diving to find the wreckage a of a massive boat. The second tract is on the ocean floor next to the wreck.
Tract #3 - Pacific Ocean
This one is a bit off (yet obviously, on) the map. Head to the large northern island at the tip top of the map. You'll find a tract here.
Tract #4 - Mount Chiliad
Marked on the map is the south entrance to the tunnel that runs through Mount Chiliad. Enter that tunnel and proceed until you reach a small set a of steps. The tract is atop them, against a door.
Tract #5 - Little Seoul
There is an old church in north Little Seoul. Head there to find the next track at the base of the building.
Tract #6 - Paleto Cove
The next tract is along the coast in Paleto Cove. In this area is the Sonar Collections Dock, a business also available for purchase. Head to the end of the dock and down the short steps to find the tract.
Tract #7 - Richman
The next tract is in swanky Richman. At the northern edge of the district is an exquisite mansion. Head up the private driveway to find a tract inside a set of stairs before the mansion entrance.
Tract #8 - Mount Chiliad
It's back to the game's big old mountain for this one. As you can see from the screenshot, the next tract is very near where you acquired the first one. Just walk down the slope and snatch it from under the tree.
Tract #9 - Downtown
You'll need a helicopter for this one. The Maze Bank is the tallest building in the game, and the penultimate tract sits atop it. Fly up to the roof for the high altitude collectible.
Tract #10 - Pacific Bluffs
In the southeast edge of Pacific Bluffs is a cemetery. In the center of said cemetery is a lake, and you'll find the final tract next to an island grave in that lake.
Congratulations - you've found all of the Epsilon Tracts!