Facebook Messenger has raised concerns about unauthorized access to user chats. Signs of unauthorized access include: receiving messages from unknown contacts, login times when the user was not online, changes to account settings such as password changes or alterations to privacy settings, chats marked as read without the user opening them, and warnings from Facebook about suspicious activities. Users are advised to regularly check their account for unfamiliar devices, monitor connected applications for unusual activity, and take proactive security measures like installing security software, logging out of all devices, changing passwords, and enabling two-factor authentication.