He nods at Sifo Dyas as they pass each other in the corridor. At this stage in his training to become a Jedi, Obi-Wan Kenobi is lucky to see a colleague as distinguished as him. Qui-Gon is always careful to keep Obi-Wan by his side. When he has moments to himself, the padawan has to spend it reporting to his other mentor, Darth Sidious. The way he understands it, he's not the only one. He's just gotten his first proof of that. The nod will be the only acknowledgment the moment ever happened.
The Sith lord is, if anything, less demanding than Qui-Gon. He asks only that Obi-Wan report in when he can, to study the Dark Side in theory if not in practice, and to keep his ears open. Qui-Gon was the ideal Jedi master to learn under, the Sith lord is always telling him, someone who doesn't toe the line the way Yoda or Mace Windu would expect or indeed represent. Qui-Gon questions everything. And he talks about things like the Prophecy all the time.
Obi-Wan doesn't know and doesn't care to know anything more about the Sith lord than what he sees in front of him. If the Sith lord has another identity then so be it. Let the Force be with him and leave it at that. The padawan minds his own council, exactly as Master Yoda is always saying. He's also patient to a fault, as even the thoughtful Qui-Gon reminds him constantly.
The darkened chamber is as it always is. He walks in casually. The Sith lord is working on something or other. Another polite nod is exchanged. That's how it sometimes is. Sometimes he must listen to a lecture and sometimes nothing at all. Really, there's no reason at all for him to come. He already knows what's in store for him; the Sith lord shared his vision of the future at the start of their relationship. He knows Qui-Gon is right about the Prophecy, and he knows what role he'll play. He will have to guide two generations of a single, brilliant family, watch as they struggle against their fates.
Already, the weariness has crept into his eyes. But he will endure. That's Obi-Wan's job.