Part of Lily's reasoning for taking Harry on for a job is self-serving - she knows that he's not going to register (partially at her behest) and she'd like to be able to keep an eye on him in the meantime. Just in case. She'd rather not this alternate-timeline son that she hasn't had slip through her fingers somehow and especially not because he's gotten arrested and tossed into Azkaban.

Anyway it means that Harry's contemporaries have been popping in and once that gets clarified it will make a whole lot of sense but for now the tinkle of birdsong doesn't elude her - it never does, however her one moment must have been too quiet to be noticed because the polite hello is enough to startle her and once Lily gets to the point where she can pause she turns toward the counter, smiling and resting her elbows on the surface.

"Hello! Sorry about that, it's a finicky potion that can't just be paused any old time. How can I help you?"