The G20 in Cannes (France) was recently saved by a Lebanese restaurant owner who found a secret file (detailing the fifth floor of the Carlton hotel where Obama was staying, escape routes, staff appointments, schedules...) forgotten by a FBI agent who had come to buy a sandwich.
The owner contacted the FBI (in the US) with his cell phone and read the file's first page to an incredulous phone operator who told him "Don't move - we're coming" and then hung up without waiting for an address. Five minutes later, two agents entered the restaurant and took the file.
Yes, they had tracked the call to the restaurant owner's cell phone - in France, thus outside the FBI's jurisdiction. Did they have a temporary authorisation from the French police force? Or did they simply ask their staff "ok you losers, who lost his file? A restaurant owner found it." "Oh shit, that's me!". This article hints at the former.