But I don’t want to tell my employer about it. Can this be discovered during a BI?
Who knows if it might be discovered during a BI? But since you intend to make money doing it you need to list it as an employment activity on your SF-85/86. Intentional omitting it would be a federal felony.
It may not be discovered immediately, but an unlisted self-employment is considered a honesty issue. An unlisted self-employment where you did can be blackmailed is a more serious (possibly disqualifying) issue.
Follow the rules, report the self-employment. You might be surprised who doesn’t care since that self-employment is legal.
Boom. What they said. How much duress it would cause if one threatened to reveal it? Vulnerability.