The lack of data visibility needed to detect the attack is caused by the threat actor gaining access to the system by known credentials. This means that the threat actor either obtained the employee's username and password through phishing, social engineering, or other means, or used a compromised account that had legitimate access to the system. This would explain why there were no suspicious logs, alerts, or failed login attempts, as the threat actor appeared to be a normal user. Reference: https://learningnetworkstore.cisco.com/on-demand-e-learning/understanding-cisco-cybersecurity-operations-fundamentals-cbrops-v1-0/CSCU-LP-CBROPS-V1-028093.html (Module 2, Lesson 2.1.2)