Alarm "change key battery" even thogh batteries are recent

Hi All.

My 2019 Fabia kept saying that the key battery needed changing. I checked them (2 separate fobs) with a voltmeter and both looked O.K.

I took the car in for it's first service and asked them to investigate. They put new batteries and the alarm stayed off, but only for a month - now back on again, even with both fobs in the cockpit.

I am taking it back to the dealer, but does anyone have an idea what the problem is? Dealer doesn't seem to know.

I don't want them to just put new batteries in, again, and only have another month's life. I do not do much mileage, especially during this Coronovirus plague.