A silent knight is someone who contributes good into the world in a way no one recognises or will ever realise. These are not so much unsung heros as unsung heros are public in their contributions and the value they provide is measured in a very middle class simplistic manner of how others perceive good deeds.
A silent knight will do what they feel is right regardless of how others perceive them and will have little interest in being credited for their actions. There may publicly demonstrate one way of living while having other ways of living which provide a greater good in a way they see as natural.
You will unlikely to know anyone as a silent knight because they are silent by nature and perhaps the backbone of human development in a way history is unable to record. It is worth appreciating those person who have improved our quality of lifes in a way we will never know.