Servicemember Safety and Security Improvement Act of 2021 This bill requires the Department of Defense to instruct each military department to review its policies and procedures for reporting members of the Armed Forces who are absent without leave, on unauthorized absence, or missing. Military installations must update the policies and procedures related to absent or missing service members with a focus on force protection, enhanced security for members living on base, and prioritizing the reporting of missing service members to local and federal law enforcement at the earliest reasonable time. The commander of each military installation must establish a protocol for sharing information with local and federal law enforcement agencies about service members who are absent without leave, on unauthorized absence, or missing. Each military installation must submit the established protocol to the relevant installation command.