OOP:Use of the "unforgivable curse"

[Ignacio]

No: HPFGUIDX 61630

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I just thought of something. Near the end of the book, harry used the 
crucio curse on that hag (lestrange), and it occured to me: how does 
the ministry of magic know when harry performed underage magic in the 
dursley's house? DO they have some kind of tracking device? Or maybe 
they only have a tracking device (kind of like tracking guns...) only 
on underage students?