Do I run the risk of getting a flat if leave USB charger plugged in all the time with no ignition? It wont be charging anything as will remove phone. But thought they still use charge in standby?

It's a furby vrs.

Thanks

IMHO if anything is left switched on/plugged in it will drain a car battery

Socket is always live even with ignition key removed. USB charger will have a 12V to 5V step down regulator, and these always consume current, even with nothing connected. It would take days, but battery would eventually go flat.