With your pistol equipped, aim at an item with the scope (monocular). As you let go of the left trigger (zoom button), tap the PICK UP button fast and repeatedly. If done correctly, you should pick up the item and have it in your inventory, this also works on dead guards. The key is that, for some reason, the icon to pick up an item pops up when you zoom out but only for a fraction of a second. Note: A future update might patch this!