If she hadn't passed the placenta you would expect to see long draggly bits still hanging from the vulva. If there's nothing hanging then it's unlikely there's a problem. Just keep an eye on her that she doesn't start to look poorly or run a temp, which would show that she had probably retained the placenta. In that case you would give her AntiBs both as an injection and as a pessary (from the vet)
Most likely as everyone says that she has devoured it or something else has. Our terriers brave the fiercest ewes to grab the afterbirth as soon as it's dropped
Our Muscovies used to (before we ate them all - birds not placentas) fight and choke over them too.