That doesn't make sense to me. There are so many things a biological father can do to "get" his child. Kidnapping doesn't seem to be the thing to do. And even if he did...why not come forward explaining - both him and Casey.
I'm just exploring ALL avenues here. If you recall, Casey told Cindy that the father didn't want anything to do with his child. Then we hear he passed away. We all know Casey is a pathological liar.
It's possible the father found out the baby was his and in desperation him or his family took it from Casey... Parental kidnappings happen all the time.
It's something to think about... it's doubtful, but possible.