The Recession Skull can be found on the 343 Guilty Spark mission. When you cross a fallen tree there are alien turrets that will explode and enemies will open fire from a structure ahead.
Don't go inside the turrets yet though and instead run to the left wall and hug it until you come to a turret with a small enclave behind it. Carefully jump down into the enclave and the Recession Skull is there.
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