Charlie is kidnapped

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Are there any stories out there were Charlie is kidnapped and in the tape that the kidnappers send to the FBI he is able to use some sort of code that only he and Don knows to tell him where he is? Maybe when they were kids Charlie made up a code for them and they would play ‘Secret Agent Man’ or some such thing.  Maybe the kidnappers order Charlie to give Don a personal message so that he would know that it was really Charlie (if it was audio only) and he’s able to say something that never really happened but let Don know that he was telling him something in code? Or if it was a video maybe he could be blinking it to him or tapping his finger? 

 
Thanks for your help on this!

Mina