Which of the following is NOT a recommended practice when the aircraft is unattended?

Removing the keys from the ignition is not typically highlighted as a necessary practice when an aircraft is unattended, especially in the context of protecting it from environmental factors or physical damage. While it is certainly advisable to secure the aircraft from potential theft, the other practices listed (tying down and gust locking the controls, putting sunshades in the windshield, and covering the aircraft) directly address the preservation and maintenance of the aircraft's condition.

Tying down and gust locking establishes stability and prevents movement caused by wind, which is critical for damage prevention. Using sunshades helps protect the interior from UV damage and heat buildup, contributing to the longevity of instruments and materials inside. Covering the aircraft appropriately shields it from weather elements, dirt, and debris, ensuring that the aircraft remains in good operating condition.

In contrast, while removing the keys can enhance security, it does not directly contribute to the aircraft's structural or environmental protection when it is left unattended. Therefore, this option stands out as less directly related to the best practices for preserving the aircraft itself.