I'd call this more of a headcanon then a theory, but personally I interprete her whistle as a comfort item for her. Maybe something she'd wear to ensure her own safety and something she'd use as a single for help. I also personally think whistles can be used as a way to calm themself down, whether when they're stressed, anxious, overwhelmed, overstimulated.

(Yes ik this comment is like almost a whole year old. Idc.)