Keyfobs normaly stored too near the car. I knew this issue. Didn’t think the distances I normally use could be classified as near. After reading again the posts I got the “ah, that’s why” feeling… Too near. Lesson learned.
Edit: bought a pair of new batteries, one for each fob, and problem solved. The good part? The warning from the car was done on a timely manner: I measured the voltage on each old battery and it was 2.9 V (the normal voltage is above 3 V); the person at the shop used an universal battery analyser and both showed “entry green”, meaning they would still perform for quite a while. I used this low battery event to learn on how to disassemble the fobs and learn how to open the car and drive it with no energy on the fob: it works. More on that later.