measuring from ground to an open wire = 0 volts?
Whenever a multimeter measures voltage it essentially does the following:


Puts a high resistance in in parallel with what you are trying to measure
Takes a very small current (that goes through the high resistance)
Measures the voltage across that high resistance


For all pedantic people, it doesn't put the resistance there, it was always there, it's the input impedance of the multimeter, the act of putting it in parallel is when you, the user, connects the multimeter to the two voltage potentials you are trying to measure.



Here's a step-by-step of (following the above steps) when you attempt to measure two voltage potentials that has nothing to do with each other.


Same as above, no problem at all. 
hmmmm how can you take some current from nothing? You can't, so you will get 0 Ampere through your high resistance. All charges will balance themselves, you will not be able to see the charge moving from one potential to the other. 
According to ohm's law we know that \$V=I×R\$, if \$I = 0\$, then \$V = 0\$. So you will measure 0 Volt. 




In order to measure anything you need to take a piece of what you are trying to measure. Had you not measured anything then they would have had some voltage potential, be it some atto Volts or a couple hundred volts. But by putting your multimeter across the two voltage potentials you also allow the electrons to move from one to the other through the high resistance. Without any voltage / current source, it will balance out and you will read 0 Volt. 

It's like measuring a capacitor, when you do so you are discharging it, so if there's some charge and some capacitance you can see on your multimeter as the voltage goes down. 

Your measurement is like measuring a capacitor, however if there's barely any charge and next to zero capacitance as in your example, then your "capacitor" will discharge almost instantly.