Hah that's funny... I'm literally sitting here waiting for a call to go meet a guy and return his wallet that I found in the middle of the street. Hope it goes well, not looking for a reward or anything.
I've had a variety of lost and found experiences over the years though. Returned a wallet with over $700 to a guy at my gym; he barely even took a second to thank me and just went on with his conversation with someone else. Returned a smartphone to a lady who was mostly shocked (and maybe a little embarrassed about some of the pictures she had on her unlocked phone...). Only regretful experience I've had so far is another time where I turned in a new (at the time) iPhone to a store's customer service counter instead of tracking down the owner myself; I left with a bad feeling that the person behind the counter wasn't going to put as much effort into finding the owner as I would.
Your feeling is one I would share. They probably won't.
My experiences are similar. People really vary in their response to a good deed directly favoring them, and I suspect the nature and quantity of deeds they do can be correlated to those responses.
I once found a lost cellphone, and after its owner called it, we arranged to meet and I returned the phone. The owner's reaction was very similar to what you describe. I refused any reward - it seems natural to me to just return it - but later that night (it was a Friday night and I was out for drinks with friends), we bumped into him in a winery, and he ended up buying me and my friends a huge jug of very good wine, out of gratitude. :)
Called the guy when I found it. He totally panicked. Was odd.
When I handed it to him, that face! Worth it.
Amazingly, he proceeded to try and give me the cash. Was an effort to decline.
At the time, I really needed a warm fuzzy. The lost wallet was a great opportunity, and I got a good one.
But, our conversation basically centered on, "who does that?"
Indeed. The stories of Japan make me want to visit just to soak a little of that in.