These three things are conducted in secret, not openly. What three? Illicit affairs, sorcery, and wrong views - says Buddha. So does this mean that secret teachings of many monks like Yogis I believe are doing one of these three things?
Buddha said that the Dhamma he teaches is not a secret doctrine...By this he meant that the Dhamma is out in the open for everybody to know...It was there even before the times of Siddhartha Gautama. The only difference is Buddha stated it explicitly so that even lay people could understand it. Although the language of the priestly class during the times of Buddha was Sanskrit, Buddha taught in Prakrit/Pali the language of lay people so that everyone could practice it. Buddha's Dhamma is 'Svakkhato' i.e. it is well propounded and well explained by the Buddha. So in that sense it is not a secret doctrine.
And regarding secret teachings, I don't know what exactly you mean by that. But to check if something is Buddha's Dhamma or not, one yardstick would be to check if the teaching helps us to
- Refrain from evil doings
- Engage in good works
- Purify our mind
Also the suttas from the Pali canon are freely available these days on various sites. So the best way to capture these posers is to keep an open, rational and analytical mind and develop our own wisdom through the practice.
It's like anything else - something that's experiential based easily gets corrupted by those without the necessary context for understanding. Let's take koans for example. Someone could rattle off the "answers" to a whole slew of them. The person receiving these answers might be able to feign an understanding, but would have no experiential basis for what the koan is actually getting at. This hampers their progress immeasurably. The fall into an intellectual rut and miss the moon for the finger. What's even more dangerous is that they might try to pass their on their spurious understanding. I think the latter is the main reason why these so-called "secrets" are guarded. It protects the tradition. I'd speculate that the higher tantras are guarded for much the same reason.