He doesn't hear anyone

Sighing in relief as there was no thundering steps or clacking of heels that would have announced one of his relatives presence, Harry quickly strippped off, not wanting Hermione's taller frame or large breasts to ruin his clothes. He debated the idea of going into his room but he was far too eager and had been waiting too long for this opportunity to wait any further. He dropped the curly hairs into the potion and waited, watching as it turned a dark purple colour.

Taking a deep breath he quickly downed it in one gulp, trying not to gag at the fowl taste. He felt a brief tingling sensation, starting in his chest and spreading quickly. He frowned at first as it started to get more intense before he suddenly hunched over, feeling his body ripple and pulse as it transformed rapidly, causing him to fall to the floor. He lay there in a daze as he felt the potion work its magic for a few minutes before all of a sudden it vanished like it was never there, leaving him feeling healthier and taller than he's ever been before. Slowly standing up, he walked over to the mirror in the hallway and prepared to examine himself, possibly now herself, to ensure the potion worked correctly