I measured the same thing here.
I guess I never noticed it before, because I don’t put the battery on the board until I actually use it. Until that time, I leave it in it’s packaging and add it to the box in which I store my finished product.
It seems a bit strange to me that the battery would be charged a bit, since this type of battery is not rechargeble. Won’t it be damaged when there is current flowing to it? I don’t considder myself an expert on analog electronics, maybe someone else who has expertise in this?