What's the low voltage displayed by a voltmeter when only one of its probes is connected?
The same thing is going on as when you touch the end of a audio cable going into a power amp and power line hum comes out of the speaker.  Your body is like the multimeter probe that is not connected to anything.  Since there are power lines in the vicinity, your body picks up a little bit of that AC signal capacitively, and that gets coupled into the audio cable which the amplifier then amplifies to drive the speaker.

Multimeter inputs are high impedance deliberately so as to minimize how much the meter effects what it is trying to measure.  Let's say the meter impedance is 10 M&Omega;.  How much capacitance in series with 220 V at 50 Hz would it take to have it see 7 V?  The capacitor would have to have about 3 G&Omega; impedance, which takes about 1 pF at 50 Hz.  1 pF is plausible enough for coupling from the loose meter lead back to the power line.