It was just during potions class. Your friend, Severus, noticed that you stopped pulling up your sleeves before brewing. It was a habit of his to observe everything around him, and memorize it (which developed from past experiences). It was quite out of the ordinary, since you've been doing it ever since you both started from first year, if his memory was correct.

And then, days went by. You kept your sleeves down, even if it got in the way and annoyed you. He caught on once you were at the library, reaching up to grab a book high on the shelf. He got a glimpse of your left sleeve slipping down, revealing a cut or two too concerning for him that he was actually considering talking to you about it.

He couldn't handle the only one he cared about to be hurt.