I have posted about the problem of "?" message to CQ at a group for CW operators in Facebook. As soon as finishing calling CQ, I often hear someone sending only "?" on the same frequency. In my view, it should never be responded since it is ambiguous and not understandable what the sender means. Some people have approved my opinion while the others tell me it is obviously meaning to send my call sign again.
The ambiguity of that "?" could be as follows;
1)The sender most likelily want me to repeat my call sign since he could not copy it completely.
2)He might wonder if I am calling CQ or not since he has just heard the very last part of CQ.
3)He could just kid me with "?".
4)He is just seeking a rare one. Recognizing me as a JA, he would quit there without saying anything to me. This is related with the situation 1). Too often I encounter this situation.
5)He would call the other station close to my spot.
2) to 5) are not pleasant cases. I would ask him to listen carefully or to quit from me.
In the case of 1), he should send me the part of my call sign as he incompletely copied.
I could smell a kind of autistic tendency, or monologous attitude in this "?" sending. That may be the main reason why I hate this kind of behaviour. CW is for communication. Why not communicating on this mode? It is not an inhuman game but a real communication between human beings. You should express your intention to give me a call in this setting.
To this "?", I would not reply anything. There is a decent and courteous manner for such a situation, which has been established by the forerunners.