If it really has no identifying marks (ie no registration) then it must be in for repainting. Alternatively there are several 747s operated by various worldwide charter companies in all white liveries, but they have registrations.
If you know the difference between the various types of 747 can you give a bit more detail ? Standard length or the short 747SP version (there are a couple of all white exec versions of this about). How many windows on the upper deck ?