Although the guard never found a thing, he continued to search the worker at the end of each shift—year after year after year. 
Finally, decades later, the man was due to retire. As he pushed his wheelbarrow out for the last time, the guard searched it, then said in despair, “Look, it doesn’t matter anymore, but satisfy my curiosity. We know you are stealing something. Yet every day I search your wheelbarrow and find nothing. How can this be?”

“It’s easy,” shrugged the worker. “I’m stealing wheelbarrows.”