[ksaua]

> surely bash doesn't cache passwords

It does if you're careless enough to type them out in clear text, e.g. when connecting to a mysql database:

    mysql -h host -u user -pMyPassword database

[ordinary]

Useful tip: with

  HISTCONTROL=ignorespace

in your .bashrc, if you start a line with a space, it won't be entered into your history.