> The Hindi should be "Their destination was secret", and the Bulgarian should be "Nobody knew where they went".

Those aren't good translations of each other (barring a helpful context); to me, "their destination was secret" states that the goal was for nobody to know where they went (with a weak implication that the goal was achieved), whereas "nobody knew where they went" states that, in reality, nobody knew where they went (and says nothing about whether that was intentional).