In the seventh book, it's stated that the Ministry know when underage witches and wizards perform magic because they have a trace on them, but in Chamber of Secrets, Dobby does a spell and Harry gets blamed for it because there was nobody else there, so do you just have to perform magic close to somebody who has the trace to set it off? But then, in Order of The Phoenix, Tonks is helping Harry pack his things and uses a spell, and there is no owl warning Harry for use of underage magic. And, how would that work in an all magic family, like Ron's, where adult wizards use magic all the time? Any theories?