iPhone USB adapter - draws current when not charging?
If the USB cord is not plugged into anything on the other end it should not be drawing any power that would be worth speaking of.

However if you are talking about the USB charger box that converts a normal power outlet to USB power to charge your iPhone then yes, those do draw power as long as they are plugged in. You can tell as they get very warm even when not charging.

It is recommended to unplug any electronics when they are not in use to save power, however I do not do this unless I am not using it for a few days.