Differentiating between Ultrasonic sensor not connected and reporting 0 distance

Hi,

I am looking to develop an Arduino platform where the sensors would be modular.

So the user could plug in ultrasonic sensors (HC_SR04 sensors in 3pin config with trip and echo connected) dynamically and arduino would know if there is a sensor or not.

Currently I have it where I just assume that there is going to be a sensor in all 4 spots. And using the NewPing library I just get a 0 if there is no sensor connected.

The Issue is that if a sensor has an object really close to it or has the closest object out of range, it will also report a zero.

I have set it up where there is a resistor going from the sensors VCC pin to an extra pin on the arduino when the sensor is plugged in and I can read that extra pin, but Id rather not sacrifice an extra pin If I don't have to...

*Is there a way to tell the difference between a sensor not connected and it reporting "0 distance" for any reason without using an extra pins? *

Any ideas are much appreciated!