You are responsible, @Cyrine-and-Yanis0 . It was your back up key that didn't work, right? It is your front door locking system that allows for someone to leave the keys inside and still lock the front door. Now, if you feel like seeing as the guest responsibility, follow the procedure for filing a damage claim which includes contacting the guest first to see if they accept the responsibility.
In my situation, I also have a back up key for the front door, which has to be locked from the outside. I am the person who lock herself out when the key I was carrying with me while on a walk with the dog mysteriously fell out of my pocket. Felt pretty dumb but was lucky to have a neighbor with a key to my house. 😛