Due to some counterintuitive geothermal reasons, it's likely that three of the four Galilean moons (of Jupiter) as well as Titan (Saturn's largest moon) all sport significant underground oceans of liquid water.
For example, despite being much smaller than the Earth, Ganymede is projected to harbor more liquid water than all of Earth's oceans combined.
(Whether this adds up to 30% depends heavily on what you start counting as a "body")
The issue isn't one of fixed vs rotation, it's that radar can't fundamentally achieve the resolution necessary to distinguish important features in the environment. It's easily fooled by oddly-shaped objects, especially concave features like corners, and so while it's great for answer the question of "am I close to something" it's not reliable for telling you what that something is, especially at longer ranges.
Note that the only proponent of the man-made theory mentioned is Graham Hancock, a somewhat infamous Atlantis-chasing pseudoscientist and overall purveyor of extremely flimsy "ancient civilization" theories.
That doesn't mean that there aren't also legitimate archeologists who might be excited about these prospects, but the lack of their mention is...suspicious.
Friendly reminder: rumors that Google is going to buy Twitch are still just that -- rumors. Nothing has been announced, and there are new rumors that Amazon is now thinking about a purchase [1].
For example, despite being much smaller than the Earth, Ganymede is projected to harbor more liquid water than all of Earth's oceans combined.
(Whether this adds up to 30% depends heavily on what you start counting as a "body")