Can you solve the MYSTERY in this story?

Can you solve the MYSTERY in this story?

This is an old story that takes place at a factory.

One day, a man left work pushing a wheelbarrow full of dirt. As he passed the security guard at the shop exit, he was stopped, and the suspicious guard searched through the dirt, looking for something the man was stealing. He found nothing, and waved the man though.

A few days later, the same man did it again, and once again the guard found nothing. This went on for a couple months. Every few days, the same man would push a wheelbarrow full of dirt, and the guard never found anything.

Finally, after several months, they figured out what was happening.

Can you figure it out?

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I was surprised that so many people got this right.
The guy is stealing wheelbarrows!

This is sometimes used as a question at security and cyber-security seminars, to try to get people to think outside the box, because that's what criminals and hackers do.
Can you solve the MYSTERY in this story?
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