In addition to a junior security engineer, a senior security engineer also does the following.
Leverage understanding of fundamental to advanced security concepts.
Constantly improve product security.
Triages and handles/escalates security issues independently.
Leads one or more security initiatives.
Conduct security architecture reviews and makes recommendations.
Interview security candidates during hiring process.
Detect and respond to company-wide security incidents.
Develop and implement preventative security measures (detection, monitoring, exploitation).
Vulnerability management - triage and manage vulnerabilities identified through scanning and manual efforts.
Identify and mitigate complex security vulnerabilities before an attacker exploits them.
Communicate risks and mitigations across multiple audiences with varying levels of sensitivity.