Timeline: Post BDM
Objects still call out to River, they need to be touched. She runs her fingertips over the dulled surface of Serenity's controls. She wonders if she can pickup on any of the ships past pilots, tap into the right frequency. Wash is still here, she expects there are others waiting for her to hear them.
They aren't ghosts, she doesn't believe in those. They'd never be classified as that, under any kind of scrutiny. Their thoughts simply got trapped, they can't change, they're merely echoes.
People remembered those ghosts but eventually Serenity will be all that is left of them.