If a person is in custody, most states don’t say an on duty cop can’t have sex with them.
So the cop just starts doing it, the victim thinks saying no won’t help, or even does say no.
Later the cop claims it’s consensual and since they turned off the cameras, it’s their word against their victim’s.
And most won’t even report it, because it’s hard to believe other cops will do anything after they were just raped while in custody, either in a police car or even in a police station.