I would imagine scuttling structures/units?

If that is right, then the other players had massive metal income from that, and it rose very sharply for all their civs as a function of time. They would have had to scuttle their entire fleet and base structures to get that kind of income. And I was Vasari, if anything, I would have had the most from reclaimation, but I had zero. They were TEC and Advent, (plus, I had already researched all the nanotech upgrades on resource reclaimation and had been in a few big battles).

If you are right, then my Vasari civ is bugged in some way, as it had a flat line zero on the summary for the entire game under "other sources"

If anyone knows for sure, let me know. Here are the items under the "Metal" summary tab:

Metal income rate
Metal available
Metal gained from asteroids
Metal gained from refinery ships
Metal gained from the black market
Metal from other sources
Metal sold on the black market
Metal sold to other players
Metal spent on ships
Metal spent on structures
Metal spent on research
Metal lost to other sources
Metal receive from allies
Metal given to allies

Would the metal from the black market infinite resource bug show up under "other"?

If they were sporadically exploring and discovering metal, then you would expect to see periodic spikes of "discovery" amounts. Their summary curves all resembled a steep upward line from the start, then around mid-game slightly leveling off to a more slowly rising, but still rising, and continuing to the end. There was no stepped activity as one might expect. Moreover, all three other players had almost identical curves, very little difference.

Didn't look like "discovery" increases at all, too steady, too consistent. I would have thought scuttle or destruction reclaimation also, except for the fact I was the only Vasari, and had zero metal income from "other sources" the whole game. But I definitely was fighting pirates and others during the game. I had the most planets, so most of my fighting was vs. the static pirates at the various planets.

TEC can get metal from "other sources" through Pervasive Economy.

I'm pretty sure that the end-game graphs don't properly represent values changing over time when the value changes infrequently. Look at the graphs like "Planets Owned", "Fleet Cap", etc . . . the lines seem to rise smoothly across the span of dozens of minutes, but if you mouse-over the graph you can see that the value was constant for the entire length of the rise, then suddenly became a new value.

. . . in that case, it's possible that you are seeing "discovery" spikes, but the spikes themselves are hidden by the way the lines are drawn on the graph.