It seems that the audit lck files are not removed during a jenkins restart or clean shutdown. This causes the "unique" portion to be incremented on each launch of the master.
Log file logger for Audit trail plugin leaks open file descriptors
Fixed by JENKINS-44129
Is there a way to update https://github.com/jenkinsci/audit-trail-plugin/blob/master/src/main/java/hudson/plugins/audit_trail/LogFileAuditLogger.java to close the FileHandler on JVM termination?