Here’s a cool tidbit of verified historical fact that you can include in your espionage game, or in an espionage type scene in any other game: During World War 2, Waddington, England’s licensee of Parker Brother’s popular game Monopoly, were approached by Britain’s defense department to produce maps printed on silk, a much better alternative to paper maps, and for which Waddington already had an established, high quality production facility. Not content to stop at just producing maps for British airman who were risking being shot down over enemy territory, they also produced special Monopoly sets for delivery to POW camps through fronts claiming to be humanitarian organizations. These monopoly sets included silk maps, files, and compasses hidden in depressions within the game board, and large amounts of various currencies hidden within the packs of monopoly money. After retrieving the supplies, POWs would destroy the remains of the game to prevent their Axis captors from discovering their ruse. Though no hard numbers are available on how many Allied POWs used these special Monopoly sets to escape, or if the tactic was ever discovered by enemy forces, but it remains one of many instances in which real life is as amazing as fiction.
True, this is just a variant of the ol’ file in the cake, but it, or a variant of it could easily prove to be the edge your PCs need to escape from imprisonment. After the PCs use the smuggled tools to escape, they still have to get through enemy territory to safety. Even then there will be pursuers to dodge and questions to answer. Who sent this? Was it really meant for us? If not, who do we need to get out with us? Is it better to get as much distance as we can or to stay close and gather intelligence and harass their foes?
On the other hand, maybe this is the sneaky underhanded technique that their foes use to escape the PC’s detainment. In this case, the escapees have to be stopped or hunted down and interrogated about how they managed to escape. Then the same or similar questions arise. Who sent this escape package? How can we prevent another from getting through? Were the actual escapees the intended target or was this all just a distraction?
Each of these questions is easily a springboard to an adventure all it’s own. An entire campaign could be built around the simple introduction of an escape tool hidden in a novel fashion.