Some basic output from SU for Confluence can be sent to the atlassian-confluence.log file located in the CONFLUENCE_HOME/logs folder of your Confluence installation.Go
Enabling SU for Confluence logging
To enable logging for SU for Confluence go to Confluence Administration>Administration>Logging and Profiling. You will see a section allowing you to add a package name for logging. Enter and Add New Entry with com.javahollic.confluence.su as the package name and select DEBUG for the logging level.
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Now, when SU for Confluence logs out messages, they should be recorded and seen in the atlassian-confluence.log file.