My PolyTune / 2 / Mini / Noir shows a "#" symbol on the display. What does this mean?
This is a symbol of the Secret Brotherhood of the Tune-O-Calypse of Doom and it is telling you that you are not playing loud enough.
Just kidding. In all seriousness, this symbol shows that your PolyTune is not bypassed and ready to give you're a pitch estimation (signal is also muted).