kawasaki er500 head gasket?

2007 low mileage,coolant level in tank dropped from between low to high by about 25mm,there is a tiny ammount of white in the oil we drained off,engine was running fine but noticed steam coming from engine when stopped could not trace the source as it was lashing it down,apart from a blown head gasket could there be any other reason for a water leak before we whip the head off.