They Didn't Miss It
Certainly, from a moral standpoint, keeping the extra money was wrong on the OP's part. In all likelihood, most people wouldn't have retained it and wouldn't find themselves on the receiving end of such a lecture.
It's worth considering the context as well. If a small mom-and-pop shop had employed the OP, the situation might have been different. However, it appears this company had ample resources, as no one seemed to notice or miss the money.
