"During World War II, as the number of British airmen held hostage behind enemy lines escalated, the country's secret service enlisted an unlikely partner in the ongoing war effort: The board game Monopoly.The maps may have been the key. The article also said:
It was the perfect accomplice.
Included in the items the German army allowed humanitarian groups to distribute in care packages to imprisoned soldiers, the game was too innocent to raise suspicion. But it was the ideal size for a top-secret escape kit that could help spring British POWs from German war camps.
The British secret service conspired with the U.K. manufacturer to stuff a compass, small metal tools, such as files, and, most importantly, a map, into cut-out compartments in the Monopoly board itself."
""It was really exciting," he said [Victor Watson, who helped make the maps]. Although it's impossible to know precisely how many prisoners escaped with the help of the hidden maps, experts estimate that about 35,000 members of the British, Commonwealth and U.S. forces who were taken prisoner during the war returned to Allied lines before the end of the war.
"We reckon that 10,000 used the Monopoly map," Watson said."