In the final episode of Stranger Things, Chief Hopper and Joyce Byers enter the upside down to rescue young Will Byers.
They find that his last known shelter has been destroyed and proceed to the upside-down version of the Byers home, where they discover signs that the demigorgon has been wounded. Then they follow its blood to the library, where they find Will.
I can formulate theories as to why they might have done these things -- the house was nearby and Will had been known to hide there in the past, and they may have supposed that the monster had physically carried Will to some kind of nest, and that it might have returned to that nest after being wounded but this is mostly supposition. Is it explained anywhere in the story why they went to the house and why they followed the blood trail?