That doesn't say much. It is an open high impedance input with some input current and of course some impedance. That leads to a voltage.
On the two 3458As here the voltage is in the range of a few hundred mV with open inputs.
On my 34401A (in high impedance mode) the voltage rises over 12V, the 34401A switches than to the 100V range which shorts this voltages and everthing starts over again.
I just checked the spec, the 3458A has >10G Ohm input impedance below 100V range.
If I switch to 100V or 1kV range the open circuit reading will normally near zero.
I have another 5.5 digit DMM and it has >1G Ohm input impedance for 10V and below, and 10M Ohm for 100V range and above. And it is reading something below 100V range and almost read a zero for 100V and above.
It seems it is really impedance related.
For 3458A mine is reading a -7V while your two are reading less than 1V, mmm...it looks each DMM has very different behavier under such condition.