Blade dropped his diary on the floor because of how shocked he was and it made a sound when it fell on the floor.
The female omega was the first one to notice Blade's presence in the room as she immediately pushed Ian away from her and ran to her clothes that were on the floor and put them on before running away from the house.
The only thing that made her super terrified is the fact that Ian is Blade's mate and she was scared of the fact that Blade is a true blood and will definitely kill both of them.
Nobody knows if all true bloods are like this or not. All this is because there is an ancient myth that says there was a male true blood who killed his fated mate who was a female alpha after finding her with another male alpha having sex. He was not the one who killed her, it was his true blood wolf who did it.