Battery life ?

Hello together, who has experience with the lifespan of the starter battery in Mondeo MK III, 130 HP TDCI, BJ 12/03 ? Reason for my question: After my car had stood almost two days last weekend, the battery was empty and it did not start any more. My neighbour then helped the engine with the help of a second battery and a starting support cable. Since then I drove every day of the last week about 50-80 km and had no problems at all when starting. I of course fear if the car is not moved again a few days the battery is empty again. Can a battery with normal use after not even three years give up its mind ? Should the point with an intact battery not be green ? At the in my vehicle it is red. (See photo in annex) greet rainbas