Using the motion sensor, you can locate the hidden audio logs & Dog Tags, & Legendary Weapons easily. It will appear as a small blue box with a speaker icon {For Audio Logs} And a small blue box with a dog tag picture}. The Weapons will appear as a small box withan exclimation mark! It is easy to miss { Check the screenshot i posted.}

With this method you can locate them all. Easily.

-----------------------------------------Update in response to user comments----------------------------------
NOTE : FOR THOSE HAVING PROBLEMS WITH YOUR GUN BEING REPLACED AFTER YOU PICK UP A LEGENDARY, JUST HIT B {DEFAULT KEY} AND YOU CAN SWITCH OUT YOUR GUNS ON THE FLY. YOU ARE NOT STUCK WITH IT THROUGH THE WHOLE LEVEL. { B allows you to change any loadout during any level..} Sorry for caps I didnt want to retype it.
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Dog Tag Sensor Screenshot



Audio Log Sensor Screen Shot



Legendary Weapon Sensor Screenshot




If you found this guide useful please