The "Alarm Footpad Sensor" warning will state "you have been off one foot pad for more than 10 seconds while riding X mph". The footpad sensor in the front has two zones. It takes weight on both zones to activate the motor. While riding over 1 mph however, only one zone needs to be engaged. This is a safety feature added into the Onewheel. The alert is telling you however that if it doesn't sense weight on that one side, the motor will disengage. While this alert is typically not a concern, you should look into shoes with better sensor contact.