Why does my Android phone think it was in Japan?
One reason could be that that location was determined from a WiFi access point, which had been previously moved from Japan to somewhere near you.

Google registers the location of each WiFi access point Android devices are connected to (if allowed in settings) and uses these to obtain a location when GPS is slow or unavailable. Unfortunately it takes some time to update the location if the access point is moved, so this kind of error can happen.