In Chapter 21 - The Unknowable Room, Tonks abandoned her duty (guarding the school) to visit Dumbledore. Upon learning that Dumbledore wasn't there she came to the 7th-floor corridor where Harry happened to be trying to force the Room of Requirement to reveal what Draco was up to.
Then this exchanged took place between Harry and Tonks:
He spun around, one-legged, and toppled over. There, to his utter astonishment, was Tonks, walking toward him as though she frequently strolled up this corridor.
"What’re you doing here?" he said, scrambling to his feet again; why did she always have to find him lying on the floor?
"I came to see Dumbledore," said Tonks. Harry thought she looked terrible: thinner than usual, her mouse-colored hair lank.
"His office isn't here," said Harry, "it's round the other side of the castle, behind the gargoyle —"
"I know," said Tonks. "He's not there. Apparently he's gone away again."
Isn't it odd that of all the corridors Tonks could be strolling she happened to be strolling up the 7th-floor where Harry was? I know JKR used Tonks as a red herring, and thus, her use of the literary parallelism (I.E. Harry spying on Draco on the train, Harry ends up on the floor in his invisibility cloak; Harry trying to spy on Draco on the 7th-floor, Harry ends up on the floor after hurting himself and his invisibility cloak slipping off him). Sure, it was probably lucky that she found Harry on the floor of the compartment of the Hogwarts Express after Draco was done with him. But are we to really believe the very same thing happened again so far out of the way on the 7th-floor?