This new icon might as well mean "low energy" because of the yellow spark.
But then I can not think of any icon that tells "you can make a factory for free..."
"I like the present. It's the only clear way I think you can convey "there is something invisible you own that you haven't used yet". "
Couldnt there be a way to make the invisible visible?
Having invisible stuff that is only represented by icons is bad design imo.
Like have the commander carry something on its back like a rucksack.
Or the beside the commander you could get a second unit, the "factory plop vehicle." That would be the most obvious, would change gameplay though.
Or have the factory tab in the buildmenu flash.
And a "first one is free!" overlay over the buttons until you plopped.