All of the collectible items are described in this Singapore walk-through.
With the South China Sea campaign completed, this mission contains 6 collectible items: 3 Dog Tags and 3 weapons. Finding the items in this mission can be tricky, so make sure you read the post carefully.
To track your process, use the Collectibles section in the pause menu.
1. Dog Tag: Horizontal Rain
Right after you reach the beach, look on the left side, you will see a small white boat. Head to that boat and look at the side that faces the sea, you will find a pinned Dog Tag.
2. Weapon: SCAR-H SV
When you reach the Changi Airport sign, clear all enemies in the under-construction building, go to the second floor, turn left twice and you will be able to get the SCAR-H SV.
3. Weapon: M240B
While you are in the city, you will have to battle against four tanks. After the battle, find the car park, get out the tank, and go to the second floor. On the second floor, you will notice three crate boxes, jump to the top of the box and jump over to the other building. Follow the small path carefully and you will find the M240B.
4. Dog Tag: Armored Column
After you have cleared all the enemies in the building, your team will jump into a small bar. Behind the bar, you will find another Dog Tag.
5. Weapon: M16A4
When you enter a jet hangar, go into the small inventory room on the left side, throw a C4 toward the boxes to blow them up. The M16A4 is hidden at the corner of the room.
6. Dog Tag: Grounded Dog
The last Dog Tag of the mission can be found in the bigger hangar. After clearing out the enemy waves, you destroy the second airplane’s engine to make a ramp from the wing. Go up the ramp and you will find the Dog Tag.
When you are finished here, continue to Mission Five.