She may well have eaten one afterbirth, or a dog or fox or badger has, or a bird has had it...
However, I have had a newly lambed ewe off colour and nothing to see, but she did turn out to have retained afterbirth. (I've wondered since whether she had a low-level infection in addition to retaining the afterbirth, and because she was low, didn't expel all the afterbirth.). Antibiotics got it out, but my delay in getting them into her made her proper poorly and we ended up topping up her lambs for the rest of the lactation.
If your instinct is that she has a retained afterbirth, you may well be right. Watch her temperature, be suspicious if the lambs aren't well fed and happy - and (despite my usually promoting 'no antibiotics without patent infection'), if you are worried, I would give her antibiotics - and minerals and vitamins too, if you can get some. Better safe than sorry in those circumstances.