Arduino Mega 2560 - Ghost Values on Analog pins
It's quite normal for unconnected ADC pins that are floating to produce random results so you're probably looking for a problem that doesn't exist. Do the readings from the actual channels used look OK? Rather than looping through the whole lot maybe try something like the following which is untested because I don't have the Arduino IDE loaded:

#define SENSOR_COUNT 2
const int analog_pins[SENSOR_COUNT] = {0, 15};

void loop() {
   for(int I=0; I &lt; SENSOR_COUNT; I++){
      int sensorValue = analogRead(analog_pins[I]);    
      float voltage = sensorValue * (5.0 / 1023.0) - 0.04;
      Serial.print("Address = ");
      Serial.print(I);
      Serial.print(" ");
      Serial.print(voltage);
      Serial.println();
      delay(5000);  
    }
}


If the channels you're interested in don't look good maybe the input impedance is too high, try something below 10K which I believe is the usual recommendation for most AVR parts. If you have a higher impedance and low sampling rate you could probably do two reads in a row so that the analog MUX has been stable a while disregarding the first reading:

analogRead(analog_pins[I]);
delay(1000);
int sensorValue = analogRead(analog_pins[I]);