A key box is a secure, hermetically sealed safe or cabinet designed to hold one or more keys, a keychain or a badge. The chosen location must be out of sight.
The right to property is in fact absolute (article 544 of the Civil Code) The law does not specify anything general. Insurers may take a dim view of key boxes because they create insecurity. It is very easy to vandalize this system in order to extract the keys to a home and access it.