There are times when a speaker makes a positive statement about a person and intends it as such, but negative and harmful information can be inferred from his words. The Torah would not consider such a speaker to be a holech rachil, peddler of gossip, since both his intent and his words are clearly sincere and positive.
Rabbinically, however, this too is prohibited under a category referred to as avak lashon hora the “dust” of loshon hara that is spread by way of carelessness.
A common form of avak loshon hara is where one sincerely praises anther person but is doing so, alludes to a shortcoming of that individual.
“Who would have believed that he would have come this far?” clearly suggests something negative about the person. Such a statement can cause the person embarrassment when said in his presence, and can be harmful to him even when he is not present. Hence, it is forbidden.
Sefer Chofetz Chaim, 3 Cheshvan, page 114