"Open the box that has the picture of your tool on it. If it's inside, stop. Otherwise, look at the object, then open the box with the corresponding picture of it. Repeat until you find your item or you've searched fifteen boxes, whichever is sooner."
By searching the boxes this way it forms a closed loop: starting with the box with the picture of their item, and ending with the box actually containing it. Had they continued on, that would just take them back to the start.
By searching in this loop, they restrict their search to only the loop that contains their instrument, and the odds of the loop being fifteen boxes or less tends out at about 30%, and although that's not perfect, it's way better than just picking fifteen random boxes and going from there.