Sirius has no idea of how someone with the brightest, most noticeable, most alive hair he’s ever seen can be so hard to find.
Though Hogwarts is pretty big, yeah, he’ll give her that. He’d use the map but James is busy with it doing who knows what, so Sirius has to limit himself to the old muggle ways.
He looks in their common room first, but there’s no one but Pete working on an assignment that he should have started more than two weeks ago, and Remus helping him with the usual patience that Sirius and James usually lack. Then he waits at the door of the girls’ dormitory for twenty minutes, and asks the first girl that comes out if Evans is upstairs. A negative response later Sirius heads to the lake, but he only manages to find Snape reading a book under a tree. After resisting the temptation to cast a spell to make the book close violently around the idiot’s gigantic nose, Sirius goes back to the castle and starts a deeper search, floor by floor.
Great hall, check. Several classrooms, check. Hospital, check.
Sirius definitely has no patience for this kind of thing.
He finds her at the library half an hour later, hunched over a book that looks heavier than the girl herself. Sirius puts on an easy smile and walks over to her, sitting on the table gracefully and looking down at Lily.
"Evans, my favourite redhead! What’s a girl like you doing in a place like this?"