The warnings plugin copies all affected source code files to Jenkins build folder, in order to show them along with the issues in the user interface. If these files are not part of the workspace of a build then the warnings plugin does not show them: otherwise sensitive files could be shown by accident. You can provide a allowlist of additional source code directories that are allowed to be shown in Jenkins UI here. Note, that such a directory must be an absolute path on the agent that executes the build.