A Nincada's eyes are useless and they spit no poisons, yet they are beloved by the ninja, for their antennae hear much more than the speakers wish to be heard. Their adept digging allows them to tunnel easily below houses, and their remarkable senses allow them to not only sense the words of someone three stories above them, but their heart rate and breathing as well. This is why it is said that Nincada hear the truth while others hear only words, although this is something of an exaggeration; even they can be fooled by a good enough liar.
Nincada are also why political and corporate secrets today are universally discussed on the fourth floor of skyscrapers or higher. Yet this is a poor precaution at best. The office buildings where open and secret meetings alike are held are full of desks covered in messy papers, and when Nincada fold up their wings, it is all too easy to mistake them for works of origami. The indoors is not nearly as dangerous to these pokemon as the open air; their wings, designed for secret bases and underground caverns, work just as well for ascending staircases at night. The employers and trainers of Nincada are not often ninja today, at least not in the traditional sense, although the clans of Fuchsia City certainly maintain their share.
Yet this era has no shortage of secrets, nor a shortage of those who will pay vast sums to learn them. There are indeed ways to keep things hidden from Nincada, such as writing or e-mail, but all of them leave far wider trails for the human spies who train them to follow. The only reliable way to keep a secret is the same in every age: you must never share it with anyone.