Hilchos Lashon Hara, chapter 3, section 4

The prohibition against lashon hara applies even if one does not explicitly identify the person spoken of at the time, but speaks of him anonymously, and from the context of the story, the listener understands whom he is speaking about. This is included in lashon hara.
Even more, even if there is no denigration in his story at all, but through his words something degrading may result to his fellow, and the speaker cunningly has intent for this result, this too is included in lashon hara and is termed “secret lashon hara” by Chazal.