The little voice inside
Your roommate told you not to eat their apples, but you want a snack, they're already in bed, and you'll run to the store and get them a new apple. They won't even know you ate one.
But a little voice in your head debates if you should do it. A little feeling in your belly tells you its wrong.
You eat the apple anyway, because you have a reasonable case - eating the apple will not harm your roommate, everything will be fine.
I've had many a situation like this, where the lack of external harm is a justification for an action that I feel I shouldn't do.
I don't value that internal voice or that gut feeling as being part of the "ethics" question, so why do I feel guilty after eating the apple?
At the end of the day, it's a choice. There may be times that I should disregard the voice, the feeling.
If I were diabetic and having a low-sugar episode, I should probably eat the apple, regardless of the voice. But if I'm just craving a snack, maybe that's not a good enough reason.
Maybe I need to value that voice, and value that gut feeling, and not use the "no-harm" argument as a justification to violate my own internal compass.
P.S. The proposed situation also introduces the need to keep a secret from my roommate (or tell them and upset them). Do secrets cause harm? Something to chew on.