Anyone know? If I at least know that then I might be able to find this mystery save file.
On Windows standalone players, PlayerPrefs are stored in the registry under HKCU\Software\[company name]\[product name] key, where company and product names are the names set up in Project Settings. (The windows registry, so there isn't any one file but a lot of entries. You can access them using regedit or other registry editors).
On Web players, PlayerPrefs are stored in binary files under %APPDATA%\Unity\WebPlayerPrefs on Windows. (When I checked the files stored there, it appears web player prefs have .upp extension).
http://unity3d.com/support/documentation/ScriptReference/PlayerPrefs.html