It’s now been definitively established that Peter Pettigrew was concealing the location of the Potter family’s house in Godric’s Hollow as their Secret Keeper.
This means that the house was invisible and Unplottable to anyone who was not the Secret Keeper (Peter), inside the perimeter of the Fidelius Charm when it was cast (the Potter family themselves), or told later on by the Secret Keeper (like Bathilda Bagshot and Sirius Black, and presumably also Dumbledore). As such, if the charm was still intact, the following people should not have been able to see the house at all:
- Hagrid when he came to pick up Harry from the ruins
- the Muggles that responded to the explosion (police/firefighters/paramedics)
- the wizards and witches who enchanted the house to be a memorial invisible to Muggles
- Hermione, when she and Harry visits many years later (and all the wizards and witches who visited until then)
The fact that they could heavily implies that the Fidelius Charm itself had been broken. Why did the Fidelius Charm on the Potters' house break? asks why that might be, but the accepted answer doesn’t really give an answer, other than that the inconsistent descriptions of the Fidelius Charm makes it impossible to know.
Jeff’s answer to the same question, however, a most ingenious solution based on this quote from chapter 17, “Bathilda’s Secret”, in Deathly Hallows:
… and now his destination was in sight at last, the Fidelius Charm broken, though they did not know it yet …
The wording in that quote is not necessarily precise enough to know for sure—‘broken’ might just mean ‘overcome’ in this case—but if taken literally, it means that the Fidelius Charm itself had been broken (i.e., undone), and by Peter himself.1 This would explain an awful lot. There’d be nothing strange about all those people who saw the Potter house throughout the years.2 In fact, almost everything that is odd and bizarre about that night and the Fidelius Charm would become clear.
Normally, though, I should think that undoing a charm or spell would be the sole prerogative of the caster—not much point in protective spells, after all, if anyone can break them. If Peter was able to undo the Fidelius Charm on the Potters', either the Secret Keeper must be considered equal to the caster with this charm, or Peter himself must have been the caster.
Is there any canon confirmation, apart from the quote above, that Peter should be able to undo the Potter Fidelius? That is, either that a Secret Keeper can always undo ‘their’ Fidelius; or, alternatively, that Peter was the caster of the Potters' Fidelius?
1 The alternative, that Voldemort could somehow undo a Fidelius charm once he was told the Secret, seems a bit far-fetched. Why would he even want to or bother? His goal was to kill Harry, and once Peter had given him their location, there’d be no point in actually undoing the Fidelius charm for him.
2 We have to assume he broke it immediately before Voldemort went there, though; otherwise someone else would be almost certain to notice that there was suddenly an extra house in the middle of the street, and there would presumably have been a fair bit of commotion about that. Even assuming this, it’s still an open question how all the Muggles in Godric’s Hollow managed to deal with the fact that a half-ruined house suddenly just popped up out of nowhere one October night. Some large-scale memory charm work would almost have to be involved there.