Dead Drops can be found on buildings attached to satellite dishes. If you press down on the left stick, Cole will "sense" all the electrical sources in the area, and if you are close enough, will also direct you to a dead drop. There is a tutorial about it in the beginning
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There are 14 dead drops in the Warrens. Below is a map if you are missing one.
It should also be noted that before you can access a dead drop, the electricity needs to be on in its respective section. So if you are in an area that is blacked out, do not expect to have access to any dead drops.
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