Does your hall sensor read correctly with a baud rate of 28800? I would try the example code exactly as provided, and change the baud rate setting in the Serial Monitor from 9600 to 28800.
Can you provide a better description or sketched schematic of your application?
Unused analog inputs will float and read arbitrary values, and sometimes read the value of a connected sensor on another channel if nothing is plugged into them. You can connect them to 5V or GND if you would like to see them read a set value instead of floating.