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As mentioned in a comment, your code has some fundamental flaws. Do not call loop() from within loop()! Aside from the code errors, your question has a legitimate element.

Testing the tri-state behaviour of an input pin will let you know if it is genuinely high, genuinely low, or floating (disconnected). The technique was given in this answer to similar problem I was encountering.

In short:

Connect a large (1MOhm) resistor to each input between the pin and ground, then for each pin:

• Turn off internal pullup;
• Read value of input pin;
• Turn on internal pullup;
• Read value again.

If the value changes from the first to the second digitalRead(), the input was not connected to anything. If the value remains the same then it is a genuine value from a connected device.