Actually, if it was in the room, housekeeping almost certainly found it and turned it in. Generally, hotels in Japan are very good about this sort of thing. They will not, however, contact you directly after your stay. You do need to inquire with them. If you remember your room number, or at least your exact check out date, it will be easy for them to locate.

Battery chargers, computer chargers, iPod chargers, etc. are probably fairly common items in the lost and found. In fact, I've even read the tip that if you happen to find yourself in a hotel without the charger that you need, just inquire at the front desk. They probably have whatever you need on hand!

Good luck!