Pondering that Secret Keeper thing

[Kirstini]

No: HPFGUIDX 77888

Alec wrote: 
>>> This is just a thought:  What if the Fidelus Charm can't hide 
you from people who already know where you are?  What I suspect is 
that James and Lily switched to Peter Petigrew as sceret keeper "at 
the last moment", but Sirus helped with the preparations of the their 
stay in Godric's Hallow.  So Sirus knew where they'd be>>>

This isn't possible. Narcissa Black must have made visits to 12 
Grimmauld Place as a child - she knows it well enough to be on close 
terms with the resident House Elf. From Kreacher's little Christmas 
visit, Narcissa and thus Lucius -> Voldemort will have worked out 
that the Order are using Number 12 as HQ. But they can't get at them 
in it, or even, apparently, in the surrounding area. Narcissa Malfoy 
could walk past her old childhood stomping ground a hundred times a 
day, *know* that it must be there, and yet not be able to get at the 
house at all without Dumbledore telling her so. Think you might be 
onto something with the bit about "guessing which house it was from 
the smouldering wreckage", though.
Kirstini