It has all the hallmarks of a low voltage aux battery.
What if you put the vehicle into Ready mode and then insert the USB key?
If this isn't a battery issue then as the USB connector is directly connected to the AV system, which is on a separate bus to the vehicle's CAN bus, in theory it should not be possible to cause an issue on the hybrid system. Obviously there are devices that connect to multiple buses and act as gateways/firewalls and flaws in the system could allow unexpected things. Luckily the IS isn't directly Internet connected, this sort of flaw is gold to hackers and the potential start of an exploit.