Good for you not letting him see you upset. I'm sure it must be devastating, not even getting the chance to sort out the cause of his unhappiness.
I think that's a terrible way to treat your wife. Just to up and leave like that. He should have spoken to you about being unhappy before just walking out like that.
That's a very immature way to deal with issues in a marriage. Talk about being blindsided.
Was he depressed or acting differently before he left?
Any major changes or life events?
Looking back were there some clues that anything was wrong?
You don't need to answer this, but did you have a good sexual relationship? Sometimes that side of things is dead and one day one spouse can't take it anymore and leaves.
Only he knows whether or not anyone else is involved. I've known people to leave while in an affair and lay low until everything has blown over, then emerge with a new partner. It's done so they don't look bad and to be accepted by the family.
If he's unhappy enough to leave like that, then he should have realised a divorce is the end result. Unless he's keeping the door open just in case....
I would not be best pleased to be dangling in your position and would file for divorce. It's like he's giving you a tiny bit of hope and sitting on the fence, so I'd knock him off the fence. It's like trying to control your future.