Digital voltmeters are so sensitive that the Earth's magnetic field will induce a very small voltage in voltmeter leads that are not touching. You get the same result—a rapidly fluctuating, low voltage reading called ghost voltage—when the leads are touching two wires that are not connected in any way.
If you suspect that a component or a circuit is open (there's a broken or disconnected wire stopping current flow), you can check for continuity using this knowledge. Make sure all the switches are closed if you're checking a circuit.
Source: Dan Sullivan, Sullivan Training Solutions, www.brighterideas.com