Do we have any knowledge about the teaching that he has not revealed to us?
There are three factors to determine what the Buddha has taught:
One is, the teaching is not limited by what the teacher can teach, but by what the student is capable of receiving. An Einstein, when being the teacher for a kindergarten, he can only teach ABC.
Two, the Buddha only responses to question. In general, the Buddha doesn't teach if not requested. Therefore, how good our classmates were able to ask the question determine what we could have in the Sutras. Example, since Ananda didn't ask the Buddha to clarify what were the minor precepts that could be removed, it indirectly caused the spilt of Samgha after the Nirvana.
Last is, some of the sublime knowledge are revealed when a practitioner reached advanced stage. By that time, the Buddha or Bodhisattva will teach the practitioner directly, as stated in some Chinese Sutras.
Could that be the teaching of tantra?
From the Original Mahayana school, which is the Chinese lineage, Tantra is categorized as non-Buddhist. By the Tang Dynasty (600CE), Buddhism in India started to mix with Hinduism, the old Indic religion in the form of Shaivism reared its head again and turned Buddhism to its host. Three famous "Buddhists" came to China and translated their "Tantric Sutras" into Chinese, they received welcoming receptions from the Emperor. However, once their texts came out the Emperor with the learnt Buddhist masters and scholars reviewed, he confiscated the "Tantric Sutras" and ordered that these texts should never be circulated because it was against the Buddhist principles, and the moral degradation and corruption embedded in them. The Emperor bounteously rewarded and gifted the three "Buddhists" then sent them away. Until 700-800 years later, in 1300CE, when China fell to the hands of the Mongolians, the Mongolian kings were followers of Mongolian Shamanism and favoured "Tantric Buddhism". These texts once were confiscated by the Emperors of the Hans were promoted and started to gain popularity. (Chinese were/are fetish of keeping knowledge bits and historical bits, they will never destroy any bits of knowledge but faithfully keeping all evidences.)
Buddhism in Tibet was first transmitted from China. In 650CE, Princess Wencheng bought the Chinese Sutras and translated to Tibetan. Until Moheyan (800CE) lost the debate to Kamalaśīla who represented Indian Buddhism, Tibetan Buddhism turned fully Tantric, mixed with Bon and empowered by Padmasambhāva (800CE), followed by many Rinpoches coming from India to transmit the Vajrayana | Tibetan | Tantra "Buddhism". In fact, only Tibetan Buddhism itself and the Rinpoches declared themselves the "Mahayana" school, as of today, 1200 years after Moheyan the Mahayana Buddhist was rejected and expelled from Tibet.
I just recount the history of Tantra the part related to Buddhism, how would you like to take it is your call.