That empty shell is quite likely from an illumination round. They make a funny whoop whoop sound as they tumble after the flare is ejected. An M48A3 tank turret rings like a freaking church bell when hit by one!!! A 155 battery was providing flares for some fire base and our company was in "area C" where the empties came down. 105 Howitzer and 4.2 mortar flares do much the same. Our battalion mortar guys, at least, had the courtesy to call and let us know to button up first if the flare canisters were likely to hit around us.