Someone entered the store during break. The spy listened carefully. It seemed the man bought a sail. This piqued the spy’s interest, why would someone need a sail in a big 20th century city? He decided to do what he did best, spy.
He saw the man take a jewel. The man almost spotted the spy. The spy never saw or heard the man buy the jewel. He caught the man’s license plate number and gave it to the police.
The criminal was caught within the week, without the spy telling anyone except the one officer he called.