Ships are giant hunks of metal and radio emitters. They light up on SAR satellites[0]. Sentinel-1 gets whole earth coverage and a revisit time of 1-3 days[1] with two active satellites. And that's the public stuff, if you can afford a fleet or even some extra fuel to steer them into interesting orbits you can get faster revisits.

[0] https://x.com/hwtnv/status/2031326840519041114 [1] https://sentiwiki.copernicus.eu/__attachments/1672913/Revisi...

There is a french company (https://unseenlabs.com/fr/) that specializes in tracking ship at sea through observing their RF emission from space. Cool tech. I'm pretty sure their main clients are not all civil...

And they also don't travel alone.

5-10 ships moving at speed across the ocean. Blasting the skies with radar.

Its as easy as anything is to find it in the ocean. And were pretty damn good at tracking ships at sea even small fishing vessels let alone a floating city.

The threat model to CSGs are basically nuclear submarines from nations that would simply tail the group if needed.