Yep, I see - the more reliable the system has to be, the more important is how I check the batteries.
By now, this is just theoretically, for something, that I might need in future. I don't always do things because I need them, but I want to extend my knowledge.
According to what is written on the battery, it is a Mignon LR6 (according to IEC), and this is what Wikipedia knows as Alkaline/Manganese battery.
So, this is a battery, you told, that does not reveal its real quality by measuring OCV. Is it a value I cannot trust on, or one I should just change the threshold to a higher value?