I remember when I was a teenager, just starting to ride, I would often place something I needed to carry on top of the gas tank if it didn't fit under my jacket (this was on my dad's CB100 which didn't have a rack, cargo net or any other way to carry cargo). I got a very quick lesson that things would easily slip backward off the tank when accelerating, or forward when braking, but it was amazingly stable side-to-side. As I've thought about it in later years, as my understanding of Physics improved, it made sense -- a motorcycle, being a two-wheeled vehicle, has to balance the vector force of gravity with the need for centripetal vector force to make a turn, such that the vector sum of the two passes directly through the center of mass of the bike and the tire contact patch with the ground. If those forces don't balance, the bike will simply fall over on the high side or the low side. If you place an object on the gas tank, that force vector will pass directly through it as well, and it won't have any tendency to fall to either side. The leaning of the motorcycle balances the load exactly as well as it balances the bike.
So it mystifies me a bit how the cable could ever "fall out" of the storage spot, especially since there's a fair amount of friction to secure it. I don't use the storage spot because I don't like the idea of the metal edges chafing on the wire, but I can't really see how the cable could fall out -- if forces got that imbalanced, the bike should crash.
I also prefer to put the cable in the trunk of my bike because it's secured with a lock. I worry about anything not locked down succumbing to a "drive-by" theft...someone sees it and takes it just because he can. Are you sure that's not what happened?