I’m the last person in the world to indulge in conspiracy theories, but once you eliminate the impossible, whatever remains, no matter how bizarre, has to be true. Melania’s disappearance is such a thing, especially since it is continuing in the face of massive speculation and commentary.
I don’t think the Secret Service would cover up domestic violence, but there is no telling how Trump might have corrupted that agency along with everything else he touches. They may also be reluctant to act without definitive proof. It could even be that they are investigating in search of that proof right now.