So I don't know how much gold it would take to rock the price, but I'm guessing it's enough as to be non-trivially "lost" however, any random flashdrive could contain any number of btc.
The analogy to gold isn't gold that was once possessed that has been lost, it is gold that is newly discovered. A new find of a big gold vein, gold on an asteroid, etc. would be a similar shock to the system.