No, in all likelihood nothing ate the legs off this star. It is a classic sign of one of two things:
1) if you have had this star about a month OR have recently done a water change, then the star is suffering from acclimation stress/osmotic shock. They are extremely sensitive to changes in salinity, pH, alk, and water conditions must be pristine, including a specific gravity between 1.025-1.026.
2) If you have had this star >6 months, perhaps 9-12 months, it is starving to death, as most eventually do.
The arms breaking off, and the general appearance of disintegration is characteristic of both situations. Unfortunately, removing it from the main tank is the worst thing to do for it, because you needed to acclimate an already very stressed animal to a new system, for at least 4 + hours. Otherwise, it makes things worse. Putting it back in the main system will probably double the stress. If the star was healthy, and this is an osmotic shock situation, then it might regrow the arms if the shock was not too bad.
Unless you have harlequin shrimp, a big hitch hiker crab, large wrasses, triggers, eels, etc...nothing ate it. Certainly not all the legs. If the brittle was involved, it was because the star was already dying and it was cleaning up, which is what it is supposed to do. Never jump to the "brittlestar ate it" conclusion, as many do, because you will likely overlook the real cause.
IMO, do not add another sand sifter to the tank. They only really do well in very large systems (long term) and are not recommended for a functional DSB because they eat the desirable critters. This is what the majority of people find will happen, eventually.
What are your water parameters?