Thanks to all,
I filled up the battrey with water, recharged it and since one week no flat battery problem anymore.
It happen to be that I may have forgotten an interior lamp on (I may have been sleeping to not have seen it before).
Nevertheless, I measured the resistance with an usual multimetre twice: once with the red lead to the (+) cable and second on the (-) cable. The value are completely different and when I tried to switch on the lamp I suspected it gave the reading of +/- 15 ohms on both ways.
May be it may help in the future.