What was going on when Nash Wells scanned Iris West-Allen, and found Eternium particles on her, early in The Flash's sixth season? At this point, nobody -- not even Candice Patton, who plays Iris -- knows for sure. During a chat with TVLine, Patton admitted that she, too, had thought about that question (after it was posed by a curious fan). Of course, the fact that it didn't seem to pay off directly might not mean it was a missed opportunity or a misfire; it's entirely possible that it was meant to play into the last few episodes of the season, which weren't shot or released.
Shortly after Nash Wells first came to Earth-1 at the start of the season, he was met by Iris West-Allen and Cisco Ramon, who had tracked him down. His sensors found that there were Eternium particles around Iris. There was a brief confrontation, and shortly after, Wells located some eternium particles in a manhole in the same location.
"I remember shooting that and thinking, 'This is cool! What does this mean, if it means anything?'" Patton told TVLine. "And I don't have an answer for that, so it will be interesting to see if we revisit that and see what that little nod was about."
Some fans, and the TVLine interviewer, had assumed that detecting the particles "around" Iris was a misdirect since they were later found in the same spot, and obviously the Anti-Monitor's subterranean lair, and finding it, were key plot points for "Crisis on Infinite Earths." Still, given the fact that Iris spent half of the season trapped within the Mirrorverse and eventually able to interface with it to some extent, it's easy to wonder whether it might be used as something more clever.
The argument against that line of thinking is just that Nash never followed it up, so we never saw a second instance that would make the audience wonder what was going on.
When The Flash returns to production this fall, it will have some tall tasks ahead of it. It needs to wrap up the dangling season five plot threads -- which include a prominent role for Ralph Dibney, whose actor has since been fired after a series of bigoted tweets resurfaced. Then it has to undertake what will essentially be a full season's worth of story, after the final season five details have been dealt with. It's just another sign of how long it will be before productions fully recover from the COVID-19 shutdown.