The antlers don't light up unless bright light shines on them since it's reflective spray not glow in the dark. So if they were being hunted in a dark forest they wouldn't be any more visible than usual. And since most predators big enough to kill a reindeer (bear, wolves, possibly mountain lion though I'm not entirely sure they live in that region) hunt at dusk and night time the reindeer aren't in any more danger than before.
'Dammit bob I can see your antlers!'