I'm sure I've read this type of story here a few months ago. No doubt,  many people also read it and the unscrupulous ones amongst us in this forum could be trying something sinister. Those who contacted you might've also read it here. You've a conscience and that's why you're doing all you can to contact your client because of the upfront they had already made. Keep searching, you're likely to find him one day. However, you must be careful because many imposters will approach on this.