Where does the 1V voltage come from?
What model number Hioki meter, which DC volts measurement range are you using, and do you have any special measurement modes enabled on the DMM like "high input impedance"?

DC voltmeters that have very high input impedance (>= 100&nbsp;M&Omega;) can and do measure residual/static/floating potentials left on metal bodies as if those bodies were charged plates in a capacitor. 

For example: When measuring DC volts, the DT428X handheld meters have an input impedance of 1&nbsp;G&Omega; on the 60&nbsp;mV and 600&nbsp;mV ranges, and the 323X HiTESTER benchtop DMMs have an input impedance of "greater than 100&nbsp;M&Omega;" on the 200 and 2000&nbsp;mV ranges.


:: EDIT 1 ::

I don't think the KEW 2200R meter is causing this. Its input impedance is 11&nbsp;M&Omega; for the DC Volts measurement function, which likely won't indicate a persistent 1&nbsp;V reading if it's connected to a small metal mass with residual charge on it.

If it's easily done, remove the relay so that you can perform out-of-circuit tests on it. With no power connected to the relay's coil, perform three continuity tests (or ohmmeter measurements) across the switch's three terminals, i.e., test the


input-output path for the normally-closed (NC) signal path (there should be continuity (less than one ohm)
input-output path for the normally-open (NO) signal path (there should not be continuity (infinite resistance))
NC output pin to NO output pin (there should not be continuity (infinite resistance))



:: EDIT 2 ::

With everything connected normally but powered OFF, try a resistance measurement from the relay's NO contact to 24V- and 24V+. It should be infinite resistance.  If the resistance is infinite, try the following:


Configure the DMM to measure DC volts.
Using jumper wires, connect a 10&nbsp;k&Omega; 1/4&nbsp;W (or higher wattage) resistor between the relay's NO terminal and ground (24V-).
With the system turned OFF, connect the DMM across the 10&nbsp;k&Omega; resistor to measure the voltage drop across the resistor. Nominally, no current should be flowing through the resistor; therefore, the measured voltage drop should be approximately zero.
Apply power to the system.
Observe the indicated voltage across the resistor. It should be approximately zero volts.
Turn off the power to the system.
Using jumper wires, reconnect the 10&nbsp;k&Omega; resistor between the relay's NO terminal and 24V+.
Repeat steps 3&nbsp;&ndash;&nbsp;7.


In step 8 if a voltage drop is measured across the resistor with the system powered ON, there's continuity somewhere between the relay's NO pin and ground. It could be the PLC software is actuating the relay (COM->NO), or the relay's socket is damaged, or a bug got under the relay socket and died and is now a resistance between the relay's NO pin and ground, or ??? (other).