The Sikarians... while my first thought was the Borg assimilated the species and are gone, it's equally possible they just assimilated a far off colony.
Alastria, which was 40,000 light years from Sikaris, is within Borg reach. They could have assimilated the technology there without actually going to Sikaris. The main one on Sikaris only worked there because it reflected antineutrinos due to the material of the core of that world. Note that when on Alastria, Kim and the woman were on a platform... obviously a beam in/beam out site for the machine to work.
The Borg obviously figured out how to make it work without that planetary core, likely by combining other races' knowledge with the trajector.
This actually would explain why Picard would remember the Borg Queen while as Locutus, despite that cube being destroyed... yes, the body itself is reconstructed from pieces but she could have gotten off before the explosion.
There's no mention of where and when they assimilated the Sikarians, so it is possible.
(Regarding them, I always wondered how long that civilization has been around. It had to take hundreds of years, probably more, to get trajector platforms across all those worlds in the Delta Quadrant. Very similar to the Iconians...)