How people who harm and abuse individuals conceal what is happening?

To conceal it well, they need to have power over victim. To gain this power they spend quite a while in early stages of relationship convincing victim they are all they have and are dependant on them. Threats are common, they instill fear in the victim.

Because the victim feels love for the person and believes they have noone else, they do what they are told and even help to conceal the abuse themselves.

Its a mind game. I know this. I was a victim. So if you think someone is being abused, they most likely are, but will not admit it because of fear. They will even defend the abuser.