He hadn’t meant to eavesdrop.
At least, not at first. He’d been working on his essay on poison antidotes (trivial stuff, he couldn’t wait to get to N.E.W.T.-level, leave the rest of these third-year dunderheads behind and learn the real art of potion brewing) when he’d been distracted by a reference in the text to something call a bezoar. He was in the library, waiting for Lily to meet up with him as they often did on Friday afternoons, but she was late, as she often was, and he found himself irritated by his own shameful state of unknowing, so he’d got up to find another book that would provide an answer to this new, itching question. His studies often lead him down such circuitous paths of learning. It may not be the most efficient method, but there was a reason he far excelled his classmates in every subject.
As he stood to begin this hunt for knowledge, he noticed something that made his skin crawl: Sirius Black and James Potter were seated only a few tables away, partially obscured by one of the library’s tall shelves, so that Severus hadn’t noticed them before. All Black would have to do was tip his chair back and he’d easily spot Severus sitting behind him.
This felt like an unbearable intrusion. This was his place, the one part of the castle where they weren’t supposed to bother him. And yet, here they were. Intruding. He wondered if they’d sat there on purpose, if that was their goal, to make sure that Severus knew that nowhere in the castle was safe from their hooliganish behavior, their constant torment that had steadily escalated in the nearly three years they’d been at school.