And even if it were then how does she know the bolt has been thrown at all? There could be a chair jammed under the handle, a million tins of tuna stacked against it, or an elephant sitting with its back to the door.
She doesn't know. All she knows is the door won't open.
The author knows and is telling us it's the bolt - and if the author is telling us as an omniscient narrator then he is perfectly entitled to tell who, how, where, when, and why the bolt was thrown if he wants.