From reading the articles related to the whistleblower SARS are not allowed to be moved or hidden. There is no policy or tool available to do so. They are permanent records.Next level stuff. I am thinking (hoping) that because thousands of Law Enforcement Officials have access to the files, it was the Special Counsel that hid the two SARs from public view, because it is something groundbreaking in there that could affect their case if made public.
Outside of that, Mueller could get a subpoena to have the SARs anytime I'd guess so someone on Trump's books removing the records permanently without any forensic trace seems implausible and/or a quick way to get some jail time until you're ready to talk.