Implementations of crypto, such as Truecrypt, rely on algorithms/ciphers such as AES which (in some modes) basically appears random... But the appearance of randomness is not enough if someone is convinced there is meaningful data there. Of course, a break such as "we can tell if there's a hidden truecrypt volume" is bad, and if I recall correctly there are ways of doing this now.

You'd need to basically never transmit the data, transmission automatically implies there is something there. If you didn't transmit, just used the data locally... And it appeared random, you'd have a pretty solid case for "they can't know". But if you slip up just once it's all over. They know.