One of the first women whom I saw storing a wad of cash and some keys in a bra was "Madame Prime Minister" to most. A pocket knife not being disclosed by such person? Wouldn't have been a problem for different reasons. Personally, I don't see how a small pocket knife in the hands of a typical passenger is worth a lot of prosecutorial resources when such item may be no more dangerous than plenty of items that are either not prohibited from being airside in the hands of passengers or that are rather readily available for fabrication airside. Would I have a different level of concern if it were a gun being stored in the bra? Yes.