Draco had taken the key with him when he'd gotten out of bed and, assuming it locked the front door of the bungalow, had stashed it in his pocket. When he had gotten to the door, he had tried locking it with that key, but it hadn't worked. Confused and a little disappointed, he had continued on to the shop. He had looked at the books Harry had brought him some more, but Draco couldn't forget the key that was practically burning through his pocket. And who cared about Muggle tools anyway? For the moment, the key and the mystery of what it opened was far more important.
Which is when he remembered some bloke mentioning not to open the luggage because it had made him punch people. On that post, in his discussion with someone, he had said that he'd found a key that morning, in bed. Closing the book with a decisive snap, Draco slammed the door behind him and ran for his little hut, barging in without noticing that the door gave a very wary creak and barreled up the stairs.
He was surprised to come face to face with Albus Dumbledore - and what a surprise that meeting had been. "Hey," he said absently, now going in search of his luggage.