Not sure if this is much help, but every once in a while my cat will do that - just start crying pitifully, usually while he's in the bedroom and I'm in the living room. I call to him, and tell him it's all right and call him to me, and then he seems okay. Don't know if he just woke up and forgot where he was, or what. It seemed to start out of nowhere and it happened occasionally for a few months. He hasn't done it in a little while. He just sounded insecure, and I went forward on that basis.