In the Yard, you will encounter multiple snippers. Reach on the other side of all these snipers to find the secret area at the end.
Secret Area #2
After getting down from the lift, you need to keep moving to the right side. In the end, drop down a level and start going in the opposite direction to find the secret area.
The third secret area is on top of the first bird portal. Use monkey bars to reach the there.
Secret Area #4
After crossing the Gantry Channel, you need to go toward the left side. In the end, cross the blast the platform to clear the way to the lower level. Drop down to find the secret area.