Are you aware of "Juice Jacking"? Don't plug in your USB charging cable to an unknown USB charging device (airports, coffee shops, etc.). It could access your data and also could load unwanted software on to your device. Only use your own charger! Or a little devise called "Smart Charge".
The French can add it to their beloved list of "astuces".
That's so French: 'don't trust anybody!', like, 'don't logon to the internet, someone might get your personal data!'
Apparently this can be prevented by simply switching the device off before putting it on to charge. I wouldn't worry too much, however, ss Brian said, the contingency is a remote one.
Saw this on CSI cyber last week.
There are umpteen sites explaining this. Wall of Sheep is probably most to the point without going into waffle. It is a possibility but in terms of likelihood apparently low. However a portable or mains device is safer anyway.