Welcome to Harry Potter Theory. Today, we’re discussing HOW Severus Snape KNEW the location of Harry and Hermione in the Forest of Dean.
In the Deathly Hallows, Harry and the trio are out hunting Horcruxes. However, things aren’t going particularly well, and that’s because they are without the fabled Sword of Gryffindor- a weapon they can use to destroy these evil items. 
Queue Severus Snape. After a considerable amount of time had passed, and the trio had managed to traverse miles upon miles of forest, Severus Snape decides it’s time to drop off the Sword of Gryffindor, which was in his possession. 
Dumbledore instructs Snape to give Harry the real sword. However, he also explains to Snape that Harry mustn’t know that it was him who presented him with the sword. This is because Voldemort could potentially use legilimency on Harry, and figure out Snape’s true allegiance. So, instead of just giving Harry the sword, he puts it in a frozen lake in the forest of dean. He then uses his corporeal Patronus to guide Harry to the sword. 
“Good. Very good!” cried the portrait of Dumbledore behind the headmaster’s chair. “Now, Severus, the sword! Do not forget that it must be taken under conditions of need and valor — and he must not know that you give it! If Voldemort should read Harry’s mind and see you acting for him —”
At this point, Harry, a worthy Gryffindor, is reunited with the relic of his Hogwarts house, and things start to get a little bit easier. 
But my question is this- how on EARTH did Snape find Harry and Hermione in the Forest of Dean?