At any safehouse buy the satellite locator upgrade with recovered salvage and get into a vehicle and start driving around. When you get within range of a radio tag a small pulsing green dot will appear on your map. Follow the map to the green dot as this indicates the location of a missing tag. Some of the tags are in difficult locations to reach so it is advised to use a dune-buggy type vehicle.
Enter a vehicle for several seconds when your health is low and your health or shield will be healed completely when you get out of the vehicle. You cannot die while getting into the vehicle and it can be done anytime during a mission.
Listed below are the 16 hidden challenges in Multiplayer which can be completed in Matchmaking, Custom Matches, or a combination of both modes. Use the 'Statistics' view to see where you are for ALL of these and not the 'Leaderboards'. 6 of the challenges must be achieved in one life while the others are overall.
1 Life: 10 Kills - 1 Weapon: Kill 10 people with the same weapon in one life.
1 Life: 15 Bonuses: Get 15 different bonuses in one life.
1 Life: 25 Bonus XP: Gain 25 bonus XP in one life.
1 Life: 5 XP - 5 Backpacks: Get 5 XP with 5 different backpacks in one life.
1 Life: 50 XP: Gain 50 XP in one life.
1 Life: Kill - 5 Weapons: Kill 5 people with 5 different weapons in one life.
10 Kills - Every Weapon: Kill 10 players with each individual weapon.
10 Wins - Every Game: Type Win 10 matches in every game type.
100 Wins: Win 100 matches (any mode).
100 XP - Every Backpack: Gain 100 XP with each individual backpack.
100 XP - Every Weapon: Gain 100 XP with each individual weapon.
1000 Kills - 1 Weapon: Kill 1,000 players with one weapon.
5 XP - Every Bonus Event: Gain 5 XP with each bonus event.
500 XP - 1 Bonus Event: Gain 500 XP with one bonus event.
5000 XP - 1 Backpack: Gain 5,000 XP with one backpack.