If as you say, you use MP3tag to view this file.mp3 and it is blank - no tags, Clearly there has to be some identification embedded within file.mp3 for Winamp to seek out a song title. The question is what identification mark does it use (extended tag)? Once you probe Winamp's forums for this critical bit of knowledge, only then can you ask this community how to use MP3tag's external sources to search a song with this mysterious embedded tag.
However, there is another way to search for a song, but this is hypothetical. It is possible to create a fingerprint for a song, much like each file has a unique MD5 sum. Basically, the software scans the song, not the tags, to create this fingerprint. Once the software has the fingerprint, it then searches a database of fingerprints to find a match (80%, 95%, 99%). Then the list of matches appear for you to select. There are software packages currently available that use this technique to compare songs to identify duplicates. They work beautifully (ex: Unknown\Untitled\Track01 = Elton John\Your Song)
This maybe what WinAmp is doing, I don't know. But all I know there is a open source project currently developing this database (ubuntu world). It would of course be incredible if MP3tag investigated the possibility of including this technology into a future version.