Thank you for your good advice. I was thinking that there is something wrong with the battery cage; now that you had enlighten me, I will postpone it.

Thanks again!

well you probably could go check any number of forklifts and find the battery bleeding voltage into the frame but in your case its not the battery case leaking voltage to the frame, it sounds to me like you have a component on the truck somewhere shorted to the frame.
I conclude this because in your description you say the voltages dissapear when you turn the truck off, only being present when the truck it switched on.

If you're not getting any errors and the truck runs ok then i wouldn't worry about it right now but it might be a potential problem that will cause an error at some point. Most electric trucks i know of use an isolated system so there shouldn't be any voltage readings to the frame at all.
But if it bugs you and you're just looking for something to do you can spend some time and start troubleshooting it.
I would start checking at the motors first and then you can go to other components till you narrow it down.

Good Luck