NI does not specify what voltage a device will read when there is no signal connected.
Since an open or unconnected channel is not being actively driven to any specific voltage you cannot expect a specific voltage to appear on the channel. To see zero volts on a given channel, there will need to be a zero voltage signal applied across the + and - terminals of the channel, which is the same as grounding the channel. To pull down the signal to zero, you can use a pull-down resistor. See Can I Use Pull-Down Resistors for Analog Inputs?
for more details.