Time rewound.
When Hermione Apparated with Harry and the others, Yaxley grabbed hold of her. Jon, who came out right afterward, was ultimately a step too late to catch up with them directly.
But this possibility had been part of Jon's calculations. He was not particularly familiar with the Ministry of Magic to begin with, and problems during the search were entirely possible. So he had warned Hermione in advance: if they were caught, they were to find an opportunity to get away and go somewhere no one knew about.
And Hermione had indeed done exactly that.
When they appeared at Grimmauld Place, she keenly noticed that someone had come along with her. Taking advantage of Yaxley relaxing his guard, she seized the chance to Apparate away with the others.
Naturally, the opportunity had been created for her by Jon, who arrived soon afterward.
Jon used mental magic to disrupt Yaxley's thoughts a little, then brought him under his control.
Otherwise, the location of Grimmauld Place would truly have been exposed.
Even if Hermione had become the Secret-Keeper, she could still ensure that the location of the house was not known to too many people. Yaxley seeing it alone and Yaxley seeing it together with the Death Eaters waiting outside were two entirely different matters.
Stephen appeared before Jon an instant later. The moment he arrived, he saw Jon standing on the stone steps of Grimmauld Place, holding Yaxley's body in one hand as though he were carrying a chicken.
Of course, Stephen knew this place. Jon was one of the Secret-Keepers too. Snape had not told anyone else where it was, but Jon had.
Besides, under the interference of the Magic Net, the secrecy of the Fidelius Charm was as conspicuous as a torch burning in the darkness.
"Take