There's more...

Windows components such as the File Server Resource Manager or products such as DropBox are using an alternate data stream to store information as well. In the case of DropBox, this stream is called com.dropbox.attributes, which seems to be used to store data about the machine the file was synced from, as shown as follows:

I hope your curiosity is now piqued, so go ahead and have a look at your files to see what lies hidden in the depths of your drives:

Get-ChildItem -Path ~ -Recurse -File | Where-Object -FilterScript {
    $($_ | Get-Item -Stream *).Stream -notmatch ':\$DATA|Zone\.Identifier'
}