The pilots reported it upon arrival via the airline's internal safety reporting process.
Aviation as an industry is built on blameless, honest post-mortems. There's a strong safety culture, and pilots are encouraged to share their mistakes so that other pilots can learn from them.
In most cases, filing a safety report will protect the pilot against any retribution from either the airline or the federal government. At worst, they'd be asked to undergo additional training. There's no reason for the pilots to engage in deception here.
In this day and age it does not seem reasonable to have less than 24 hours of CVR storage - it gives time for any flight, and also space for people forgetting to pop breakers.