I was recently reading about a case where part of the evidence appears to be a deleted bash history-file. From what I gather, the accused says that the removal was a clean-up job to remove inadvertently stored passwords rather than an attempt to hide nefarious activity.
What had managed to pass me by in 15 or so years of using bash but not reading man bash properly is the HISTCONTROL variable. If it is set to ignorespace then commands entered with a leading-space will not be stored in the history. Like all the best discoveries I found this only by accident on a machine where it was turned on.
It's been quite useful in keeping my history pruned. However, not usually for security reasons around hiding passwords — passwords on the command-line have other problems and I'm sure any security person would tell you not to use them just from a defense-in-depth perspective. That said, from a "keeping ctrl-r reversi-search useful" point-of-view it's often helpful; for example I've mostly trained my fingers to <space>rm -rf because I'm sure I'm not the only one who has deleted the wrong thing via a history-recall-combined-with-typing-to-fast scenario.
So, HISTCONTROL=ignoreboth (which also ignores duplicates) is certainly a useful one to slip into your .bashrc. At least it would be one-less thing to explain to the FBI!