In The Deathly Hallows, Hermione accidentaly leads Yaxley into 12 Grimmauld Place. After this, the trio abandon the house, knowing that after Dumbledore’s death they were all secret keepers, and now Hermione had revealed it to Yaxley — “Even now, he could be bringing other Death Eaters in there by apparition.”
However, in Book 3 when Professor Flitwick explains the Fidelius Charm (secret keeper) he says it is “an immensly complex spell involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth imnpossible to find – unless, of course, teh secret-keeper chooses to divulge it. As long as the Secret-Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years…”
— Apparently, it is only if the secret-keeper divulges it that one can know; even somebody to whom the secret keeper revealed it to cannot divulge. How, then, would Yaxley be able to show the other Death Eaters if he himself was not a Secret Keeper?