Security best practices in Splunk app development involve multiple approaches to ensuring secure code and infrastructure. Implementing security in the software development lifecycle is crucial as it ensures that security measures are integrated from the beginning and throughout the development process. Using a dynamic scanner such as OWASP ZAP to scan web application components for vulnerabilities is also essential as it helps identify and address security flaws that could be exploited. Storing passwords in clear text in .conf files is a poor practice and should be avoided. Manual testing is important but relying solely on it without implementing a comprehensive approach, including security in the development lifecycle and using automated tools, is insufficient.