It's possible they are moltings, but I have to say that I found this morbid discovery a few times and this wasn't the case; turns out that I had blue leg hermits which were attacking & killing the scarlets when they came out to change shells (which they must do every so often, and must be provided with empty shells to minimize competition during this vulnerable stage). If you know exactly how many you have, and none are gone, I wouldn't worry about this being the case.