@Bobby *******
I think it's up to the discretion of the officer. I wasn't offering advice, but just a comment of my past experiences. As I had indicated that I don't know whether being truthful of saying 'no definite plans' would have gotten me in trouble, since I have never been asked for proof of a way to leave Thailand.
I totally understand the purpose and officer's inquiry, and I wonder if someone holding an elite visa would also be required to show a return flight? Do enlighten us.