To disciples of increasing purity, ability, and rarity the Buddha gave more private guidance in the subtle mysteries. It appears that such teachings are included in the Mahayana sutras. There is no certainty, however, that all of the tantras were taught while the historical Buddha was alive. To an extremely small number of pure disciples the Buddha could appear today. They could encounter Vajradhara, the King of the Tantras, and he could reveal tantras and quintessential guidance to them.
This is possible even though more than twenty-five hundred years have gone by since the historical Buddha passed away. There is no possibility, after the Buddha’s death, of additions being made to his public discourses. But I think that teachings to disciples of pure action do not necessarily have to be given during the historical Buddha’s lifetime. (pg.44)
–from Transcendent Wisdom by H.H. the Dalai Lama, translated, edited and annotated by B. Alan Wallace, published by Snow Lion Publications
(as quoted in Snow Lion’s “Dalai Lama Quote of the Week” on June 3, 2011)