Hmm... let's see, explanations for strange artifacts and super-robots. Even without aliens, there are lots of them:
* Lost technology. Officially, technology overall has advanced since the Night of Fire, but given the extent of the devastation there's always room to say that the results of such-and-such a science program were forgotten after producing some super-item.
* Prototypes. They could be given to the player for helping get things needed to research them ("Of course, it will be years until a commercial version hits the market, but here's a prototype to play around with! Come back from time to time and tell me how it performs.") There could even be a 'quest' consisting entirely of a scientist who gives the player a potentially dangerous prototype of some sort to play around with and report back on. If it doesn't kill them, they can keep the prototype. Or the player could steal one, of course.
A lot of the lesser anime super-robots fall into this category, don't they? The hero is given the prototype of some totally new super-advanced mecha to play around with or something.
* Lab destroyed. Space, when you think about it, makes a perfect spot for secluded, dangerous testing of experimental concepts. It also makes a perfect spot for an experiment to go wrong and kill everyone present, leaving a dead research station for the player to explore, with a circle of dead scientists standing around a eerily glowing (but totally harmless, right?) artifact. The player could be sent to investigate the station after communications die, and could choose to pocket the artifact and say nothing about it in their report.
* Secret technology. Maybe there's a family out their with a robot that's been past down through generations, based on a technology that nobody else has been able to duplicate. The leader of a science-based faction might have such a mecha... maybe their grandfather killed the scientists who developed it to keep its secrets from getting out or something. And what decent military wouldn't have secret technology of some sort or another?
* Banned technology. Biotech robots fall into this category. Maybe there's a robot out there that runs entirely on fresh embryos or liquefied people or something. Of course, what's banned in one place might be legal somewhere else, so this is more for flavor than artifacts.
* Impossible to reproduce. There might be some items whose technological secrets, even if understood, simply cannot be duplicated, or at least not mass-produced. In anime these are usually 'mystical' robots powered by a human soul or something like that, but they don't have to be. There could be a robot whose functioning depends on a fragment of an extremely rare element, for instance, one that was found on a stray astroid that was used to produce it but is not known to exist anywhere else in the solar system. This could also extend to items that are restricted to a small subset of users... perhaps only certain bio-engineered people could operate it, or those with absurdly high scientific or mystical abilities (some sort of 'mental amplifier'). Such things would effectively be artifacts, since there would be no purpose to mass-producing them.
...I still want a robot powered by rock & roll. That would make the performance ability much more useful. Come on, surely the big televised dueling leagues would go for a robot called the Hendrix that attacks with a guitar?
