He got lucky, Tsunade was on her way out from the village.
"Lady Hokage!" Kakashi yelled.
"Kakashi? What is it?" Tsunade said turning to him.
"No time. We have to go." He said, turning and waiting for her to follow him.
"What's going on?" She asked again, not moving from her spot.
"Someone is here for Brook. She knows about the demon now." He explained.
Kakashi ran back to the clearing, but no one was there. The only thing in the clearing was a kunai knife on the ground. Kakashi was the one who noticed the blood.
Kakashi summoned his ninja hound, Pakkun, the talking one.
"Is it her blood?" Kakashi asked the dog.
"Yes. It's Brook's." The dog said.
"It can talk!?" Tsunade exclaimed.
"Can tou track her?" Kakashi asked, ignoring Tsunade.
Pakkun walked the perimeter of the clearing trying to pick up a scent. He shook his head at Kakashi, who was ready to just start running for her.
"Kakashi. There's nothing we can do." Tsunade said gently.
"We have to do something! I can't just let them take her!" Kakashi screamed at her.
"We'll get our trackers out here to look for traces. That's all we can do right now."
"Then get them out here. Now." He jumped into the trees looking for signs of anyone's presense.
He returned home four hours later, on Tsunade's orders. He had run himself ragged.
He passed out on the couch, dreaming of her all night.