The Traces Remain.

This is a typical story from the 'cold war' era. A group of kids play on a quiet street in Sofia. From a nearby window falls a key. The helpful youngsters quickly return it to the owner, but in the haste switch the keys. The unintentional error disentangles a whole spy story. The young detectives doubt the good intentions of the man they tried to help. It turns out that the key is from a secret apartment where saboteurs are hiding. The secret network is exposed to the authorities and the kids feel like heroes.