I think below is the line which you want to understand
However, when, after the financial statements have been issued, a fact becomes known to the auditor that, had it been known to the auditor at the date of the auditor’s report, may have caused the auditor to amend the auditor’s report
it means after signing audit report if he comes to know any FACT from pre signing period, which if known on date of audit report , auditor would have changed his opinion then he is suppose to discuss it and for revision of financial statements.
for example there existed big unaccounted employee claim from Jan but auditor came to know about it after signing audit report.