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To be fair I don't know of any passwords my wife can't know about. But I know that's barely an argument :-)

Why do you assume that I have saved passwords (or anything else) on my computer to which my wife is not permitted access?

For the sake of argument, I'll answer this with s/wife/friends/

Yes, if I'm not besides it, I will switch user accounts. And if I walk away from my computer for a bit, I lock it (except when there are no untrusted people around, like at home). I have "Lock Screen" bound to Ctrl-Alt-L, so it's trivial to do.