Have you tried Visual Studio? I am not sure but sometimes i was accidently opening wrong file during my data search times and i was seeing game interface and icon things etc etc.
I actually got no idea how to open those files there.
BUT in the data.pak (under data.pakMapObjects_(AdvMapObjectLink)) there is the exact same folder. It actually contains every file that's also in the Editor/ IconCache folder.
There those files are simply xdb-files, which can be opened as a text and are all of this structure: