While evaluating security at a Fortune 5 company, a penetration tester was suspicious that the small-town restaurant across the street served black duck eggs, a rare Chinese delicacy. The restaurant was reported to the FBI and was revealed to be a front for a Chinese espionage operation.
Julius Caesar would personally conduct espionage on his enemies. Once, he even dressed up as a Gaul and snuck behind enemy lines. When his soldiers failed to sneak him back, he boarded a Gaul ship blockading the Romans and sailed back into Roman territory without anyone noticing.
North Korean spies stationed abroad have to receive training on how to shop in supermarkets, use credit cards and visit clubs, as these amenities are not commonplace in North Korea.
In 1945 a group of Soviet school children presented a US Ambassador with a carved US Seal as a gesture of friendship. It hung in his office for 7 years before discovering it contained a listening device.