1) Ethical hacking involves identifying security vulnerabilities in systems by simulating techniques used by attackers. It typically involves footprinting, scanning, gaining access, and maintaining access to target systems. 2) Social engineering techniques like pretending to help retrieve forgotten passwords can be used to gain unauthorized access to user accounts. 3) Trojan viruses allow remote access and control of infected systems, enabling hackers to maintain long-term unauthorized access covertly. Finding unexpectedly open ports or information flowing from an organization could indicate a Trojan infection.