Where is Buddha now?

This may seem, at first sight, to be a question of purely academic interest. But the more one thinks about it, the bigger the implications.

I mean, legend has it he (that is, the man who up to that moment was known as Prince Siddharta Gautama) reached Enlightenment after spending a considerable amount of time under the One Tree in Bodhgaya, Northern India.
Upon reaching Enlightenment, he was thus qualified to break the chain of births and rebirths, and enter Nirvana.

Now, here comes the problem. What is Nirvana? Is it Heaven? Paradise?
No, it's not supposed to be a place like that. Nirvana is supposed to be... nothing.
Nothing.

So the next question must be:
How do we know that Buddha in fact did enter Nirvana?
Certainly nobody else could have told us - us, here, meaning his followers in 500-something B.C. Because if somebody else told us, if somebody else was the first person to enter Nirvana and then watched Buddha come in, then that person would have been the Buddha.
The only person who could have delivered that message must be Buddha himself.
But how on Earth (excuse the wording) could he have told us, if he was... nothing? It's simply not possible!

That means, if Buddha somehow left the message "Hi guys, I have now entered Nirvana" then he must be a liar. That message must have been sent from somewhere else. And that means that he is still here, among the rest of us!

Okay, somebody may argue, there could be another way. The message could have been sent through a bodhisattva - a person (a vehicle) so developed, so high-level, that he or she had what it takes to enter Nirvana, but chose to return down here in order to help the rest of us.
That bodhisattva could have said "Yeah, I saw him. I went to Nirvana... or to the gates, anyway... and I saw Buddha there. I'm quite sure it was him."

But I don't think we want to settle with hearsay evidence.
And the problem in Buddhism is, there is no God who could have stepped down and show us a miracle.
For how do you step down from... nothing?