My underboss got taken, and was released shortly after. He still shows hours later as being imprisioned...

He is in heaven. Or hell if he didn't change his ways. No just kidding, so if he doesn't appear back in your turf after 24 hours then I would worry and make a customer service ticket. But sometimes that just happens even to me. I will pay ransom and then it will show him literally limping out of their turf wearing like a straight jacket or something but then on my end he's still behind bars. And then hours will go by and I'll check and there he will be. So my guess is that by the time you read this if you EVER come back and read this, your underboss will have long since returned and possibly been captured again multiple times haha. Hope this helps.