Secret Patrol Missions are a special type of Patrol Missions released in The Taken King expansion (Year 2).
General Information
When you're on Patrol on any destination, there's a chance that you'll just automatically pick a secret mission up. They start off with an objective description that's looks like ????????. Each mission has 3 steps and each step requires a different action, and they've got decent patrol rewards for completing them.
Secret Patrol mission has about a 10% chance to automatically spawn following the completion of any normal patrol mission. I like to get mine by doing kill and VIP patrols, but you can really complete any patrol beacon and it will have a chance to give you a Secret Patrol mission when you finish. Expect to do about 5-15 patrols before you get one.
Reward
For completing all three steps, you get a bit of reputation, a chunk of glimmer, possibly a Rare Engrams or two, and a guaranteed 2 to 3 Strange Coins.
