Not sure if it’s the most efficient but check each chest you find. Especially the ones you get the special keys for. You can only hold one key, so once you get a key, go open that chest and open the spot up for another key to drop. Also three special chests open after each containment, they will be in random spots, but there are only a few spots, so once you know where they are, they are easy to find after the event. Good luck.