This page in our guide to Days Gone focuses on the secret ending. Luckily, the hidden ending isn't problematic to unlock. You shouldn't worry about missing it, either.
The first thing you have to do is to complete the main storyline, that is HUGE SPOILERS eliminate Skizzo and Col. Garret during the assault on Wizard Island.
Keep traveling around the world. After several dozen minutes, O'Brian will contact Deacon over the radio. The man asks you to meet him in person - this unlocks a new mission called There's Nothing You Can Do.
Go to Pioneer Cemetery. This place is located in the north-east corner of the Cascade region. You can fast travel to the nearby NERO Research Site.
O'Brian waits for you near the entrance to the cemetery. Approach him - this will trigger the secret ending which has an interesting twist.
This mission ends Finding NERO plotline. Deacon will receive IPCA Stun Gun crafting recipe. This unusual weapon stops and sets a Freaker on fire. Check List of crafting plans page to learn more about this weapon.