Is it lashon hora to unintentionally tell Reuven about a negative experience you had with Shimon while Reuven never heard of Shimon before and has forgotten Shimons name the next hour? All Reuven is left with is a story of negative experience but no name
The Chofetz Chaim says that if we say the name of the person it is considered lashon hora. He does not differentiate whether the person who hears it knows the person or not. It might not be as bad as when the person hearing it actually know the person, nevertheless it is still considered lashon hora.