Just more subterfuge.
Not knowing if and where they found the small bones of her hands, the ring might have washed away, fallen away, or been pick up my an animal (many small critters hoard such finds) and transported away.
Plus, the missing ring wouldn't really be important for "staging"...after all, if they are going to the trouble of binding her hands and killing her, the :stager" sould want it to look like a sexually motivated murder. Taking the ring or not taking the ring wouldn't really be that important to staging such a scene.
The simplest answer is - no staging. She was assaulted and raped and killed and the killer took her ring or it has, through natural means, been dispersed from the crime scene.