His survival in part demanded being aware of his surroundings. Although he thought the house was empty, he was still prepared for it not to be. There could be Heartless, or monsters, or just stray animals. So as he made his new bed, he did sense that he wasn't quite alone.

It might only be a figment of his imagination, but he knew he better recheck the room. If he intended to sleep here later that night, he had to be sure it was completely secure. Tucking in the corner sheet tightly--military corners--he went to one window in order to open it more for more light.

The fact that she was sneaking behind that very window was an unintended bonus in getting answers more answers about this new world.