"de haber sido"?

[AngelicaDeAlquezar]

There are two ways to say the same thing:
...de haber sido presenciada... = ...si hubiera sido presenciada...

If you check the dictionary, entry #12 presents "de" as a conditional when followed by an infinitive.

- De haber sabido, no vengo. = Si hubiera sabido, no habría venido.
(Note that in the first sentence there is a colloquial usage of the present tense as a conditional conjugation.)
If I had known, I wouldn't have come.

- Vendí mi coche el mes pasado. De saber que tú lo querías, te lo habría vendido a ti.
I sold my car last month. If I had known that you wanted it, I would have sold it to you.

- ¡Qué sorpresa que estés aquí! De saber que venías, te habría preparado una fiesta de bienvenida.
What a surprise that you're here! If I had known you were coming, I would have thrown a welcome party for you.