OoP: Harry and Decree for Underaged Sorcery

[M. J. Pascual]

No: HPFGUIDX 65618

An idle thought that I had while skimming through OoP for the 
*mumble*teenth time.

[Obligatory spoiler space]
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What's to prevent Harry, or any of the other kids for that matter,
from performing magic at 12 Grimmauld Place?  Dumbledore has the
place hidden via the Secret Keeper charm, so nobody can find it
unless he tells them.  That means no pesky messages from the Ministry!

"I'm sorry, if I violated the Decree as you claim, why didn't I 
receive an owl like the other two times?"

:)

--Mark