Stilo flat battery

With the engine off, put a low wattage 12V bulb (interior, side repeater, stop/tail or similar between the +VE battery post and its clamp. If the bulb glows pull fuses until it goes out. Then you will be able to determine the offending circuit. Could be the alarm system. If it glows but doesn't go out, get the alternator checked out, or if mecanaical lift its heavy duty connection, then if separate its control connector that feeds the voltage regulator. Do not attempt to start or run the engine with the lamp in series, but in any case the engine won't start with the lamp in series. An engine running with no battery connected will expensively fry the electrics. This should be your first, cheap and easy, start on diagnosing the problem. If the above fails, we get on to has the problem always been there, when did it first occur etc? Also to progress further you would a meter to measure any drain on the battery, I'm surprised this wasn't done when you had the battery checked.