Core answer: there are two extra story beats. The default post-credits scene (Sabretooth, Apocalypse, Horseman of War) plays for everyone and is the X-Men sequel tease. The secret ending is a separate farm memory unlocked through the Nightmare Cabin. If you only watched credits and left, you have not seen it.

1Finish the Main Story First

2Clear All 14 Nightmare Doors on Bronze

3Return to the Nightmare Cabin

4Leave the Cabin and Follow the Scent Trail

Key Decisions

DecisionWhat to do
Chase Adamantium on every trial?No, not for this ending. Bronze on all 14 plus Bronze on Best There Is is enough.
Do trials before beating the story?You can. The 15th trial still only appears after the campaign.
Watch credits and quit?You will only see the Sabretooth/Apocalypse tease. Go back to the cabin in free roam.
Missing one door in Madripoor?The secret ending stays locked. Count 14 carvings before starting Best There Is.

What Each Ending Sets Up

Common Mistakes to Avoid

Checklist Before You Stop Playing

If the trophy has not popped, you are still missing a door or the trail, not a hidden button prompt.