Where's the Macaroni?

I was recently in the Pathmark of Harlem supermarket and found an empty one-pound box of Creamette elbow macaroni on the shelf alongside full boxes. Actually, I shook the box and heard only one macaroni inside. The box (with a window so you can see the product) was sealed so it came from the factory that way. I watched the associate for a few minutes to see how long it would take him to discover the box. About three minutes later, he saw the box and tossed it in one of the empty cardboard boxes on his dolly. He continued to refill the shelf with boxes of macaroni.
It was the first time I'd seen an empty box of macaroni on a supermarket shelf. If I'd had a phone camera with me, I would have taken a picture of the box because it was such a novelty. It made me wonder how often that happens and how diligent Quality Control is at the manufacturer's end to prevent such an occurrence.
I've written to companies whenever there was a missing item in a windowless box like six coffee packets instead of seven and received a letter of apology and a coupon as compensation. But how does an empty macaroni box with a window leave the factory undetected?