After her disappearance, Sax thought he saw her in a blizzard (or maybe sandstorm), and remains convinced that he really has. But she is never seen by anyone again. What really happened with her?
I think that what happened to Hiroki is that she becomes one of the Mars myths. This is the real intention of the author.
The difficulty to assert if something is real or myth is, in a sense, what science is about. In my opinion this is an important theme for K.S. Robinson. This is especially obvious in his book Icehenge.
In other words, my interpretation is that nobody knows what happened to Hiroki (even Robinson) because, he was searching to elicit in his readers this feeling of legend mixed with real character that makes you to ask the question in first place.
Given the extreme circumstances that Sax found himself in, it is likely that he was simply hallucinating. The accounts of the Zygote survivors all indicate that Hiroko and a number of her inner circle were killed by the metanational forces.
Still - there were other rumours and apparent sightings, but as Mars became more populated no evidence of a new hidden colony was ever found. The first hidden colonies could only get away with hiding in the unpopulated south pole, so it seems unlikely that they could continue hiding once the Martian population started to grow and spread.
In the very end of the entire series - very end - Ann comes near to death, and then sees an older Asian woman flying a kite at the sea's edge.
This is only speculation, but there seemed to me an enormous significance of birth to the closing being a combination of: 1) New birth (Ann & Sax together with children), 2) Blue Mars realized, 3) The horticulturist 'mother' of Mars Hiroko flying a kite (was it her?).
It seemed to be poignant to the author to point out the ethnicity and age of the last stranger seen in the last pages of the last book, by a couple who fought at extremes, to be in fact a very older Asian woman from Earth who was flying kites with kids. Life, birth, growth - that was Hiroko Ai.
Since there's no concrete answer, this is my speculation. I see Hiroko when I read those last couple pages. Flying a kite while Ann and Sax are at the beach.
I think it's safe to say Robinson intentionally left it unanswered, though he I believe he had a most likely outcome in mind and tipped it off. It is significant that Hiroko had reached mythic status, sometimes being seen in two places at once. I don't think the Asian woman referenced at the end could be Hiroko, because she would have been recognized. She might have been a nod to the other false Hirokos like the one on Earth. To be honest, I am not sure where he was going with that, because it was hard to read it and not think of Hiroko (one final tease for the reader perhaps).
My own view, sadly, is that Coyote had it exactly right. Hiroko was killed in the initial attack and "disappeared" the way secret police have done time and again. No mystery to solve, just a grim and abrupt end. Sax hallucinated her and his later enhanced memory was just an echo of the hallucination. I would add as supporting evidence the fact that she did not even attempt to reconnect over a long series of funerals for the first hundred.
I find this ending unsatisfying but realistic. It is how I would read the same account laid out as non-fiction and see no reason to apply different logic here.
I would also make the case that Coyote's statement marks a significant development in the book. In the following passage, Robinson is tipping off the reader that "Mars exceptionalism" doesn't always hold, and sometimes the explanation can be as simple and brutal as any earth-bound banana republic. He does this through a high-credibility character (Coyote) who finally comes out with the point everyone else is afraid to voice (from Part Nine, Natural History):
Coyote shook his head violently. “No. I am the exception that proves the rule. Anyone else, when they are reported in two places at once, that means they are dead. A sure sign.” He made a stop thrust to forestall Sax’s next remark, shouted “She’s dead! Face it! She died in the attack on Sabishii! Those UNTA storm troopers caught her and Iwao and Gene and Rya and all the rest of them, and they took them to some room and sucked the air or pulled the trigger. That’s what happens! Do you think it never happens? Do you think that secret police haven’t killed dissidents and then disappeared the bodies so that no one ever finds out? It happens! Fuck yes it happens, even on your precious Mars it happens, yes and more than once! You know it’s true! It happened. That’s how people are. They’ll do anything, they’ll kill people and figure they’re just earning their keep or feeding their children or making the world safe. And that’s what happened. They killed Hiroko and all the rest of them too.”