We also had a really good experience with the tenants and we are really sorry that such an unfortunate incident occurred. However, when we give the keys to a tenant, they become solely responsible for securing the house, no one else. The girl who changes the sheets and the towels has a set of keys, just in case the tenant needs in an emergency, but these are never supposed to be used by her. The girl was never informed that the family were leaving (or had left), so when she finished she left the house the way she found it.
We weren't there when the police arrived, but we were on the phone with the girl and we heard a lot shouting and accusations against her, up to the point of making her cry.
I understand how stressful the situation can be, that's why my parents drove all the way from Madrid to try and help as much as possible. However, from our point of view, the girl is trustworthy and has been with us for a long time without any issues ever happening.
The ad doesn't mention a safe because the house itself is completely secure. All of the windows are security glass and have burglar resistant shutters, therefore if all doors and windows are secured there is nothing to worry about.
Just to mention one last time that after check-in the responsibility for securing the house is with the tenants, and no one else.
We are deeply sorry that this situation occurred, and despite the incident, the overall experience with the tenants was great.