In Deathly Hallows Hermione says that she brought a Death Eater within the Fidelius Charm's protection of Grimmauld Place through Side-Along Apparition. Since she was a Secret-Keeper, she's shared the secret with him. This is a major plot point: they can't return to Grimmauld Place, nor can they call Kreacher for help, because they believe it would help the Death Eaters find them.
Her guess isn't discussed further in the story. The narrative heavily implies that Hermione is right. But reading the Pottermore extract about the Fidelius charm on another question:
The Fidelius Charm is not without its weaknesses. If the Secret Keeper wishes to do so, they may divulge the information at any time (although the secret cannot be forced, bewitched or tortured out of a Secret Keeper who does not wish to give up their secret; it must be given voluntarily). (Secret-keeper, Pottermore Book 3 Chapter 17 Moment 2)
She definitely didn't give up the secret voluntarily, and in fact, was trying to get the Death Eater to let go of her clothes until the last second.
Does this mean that Hermione didn't actually bring a Death Eater within the charm's protection, and they wouldn't have had to suffer as much if they'd taken a risk by, say, having a quick look at the house or at least calling Kreacher?