Anonymous gestures almost always land wrong, especially at work. They can feel unsettling instead of flattering, because she has no context and no choice in the interaction. What feels gentle to you can feel heavy to someone else.
If you really want to say something, keep it simple and not secret. Something like, “Hey, I don’t mean this in a weird way, but you have a really positive energy. It made my week.” Then let it go. No follow-up. No expectation.
And if even that feels like too much, it’s okay to keep this one to yourself and let the songs be the place it lives. Not every feeling needs to be delivered to be real.