It's perfectly believable. Whether it is more believable or not is a toss up. If you employ such a large number of people there are bound to be a couple of bad apples, and unless you have very good internal processes and monitoring it isn't all that hard to imagine someone doing something they shouldn't be doing. But absent hard evidence that it happened that way it interesting speculation but no more than that, besides, it can be impossible to distinguish between the two even if you have evidence of an inside job that looks like incompetence!