Battery shouldn't be going flat. More likely there was no oil in any part of the engine that was of higher potential than required, as well as no fuel in the lines and all the other things that find new low points under gravity. This causes the starter motor to have to turn more times than might otherwise be required. If the battery was measurably flatter, I am pretty sure you would find that it would simply go 'tick, whirrr' and not actually do the thing where it starts.

You should also check the oil and coolant levels in cars that haven't been started in some time (as they might have decided to vanish). You should also check oil and fluids in your own car.