Hilchos Lashon Hara, chapter 3, section 2

When we sometimes find a leniency in Chazal that one may speak if one would not stop from speaking the lashon hara even before him, this deals with an ambiguous matter that can be understood as derogatory or otherwise, depending on how a person expresses himself when he speaks. Chazal say that if he would say such a thing before him, this proves that he does not intend to denigrate him since a person is generally embarrassed to shame someone to his face. But [if he would not speak it] before him, even if the derogatory interpretation is only dust of lashon hara, it is nonetheless forbidden.