Therefore their clicks would have two functions: to make echos reflect from objects around them, and to imitate those echos later on?
Not sure if those ideas came from then current research into dolphins, or if Brin took some artistic liberties and came up with them.
Therefore, to imagine an object they have to:
1) have the ability to create unique shape-sounds with their vocal clicks and
2a) had to have either seen/heard the shape before, OR
2b) have the ability to imagine what an imaginary object would sound like.
If research shows the way of conveying imaginary objects is 2a, that would mean dolphins are physically unable to convey abstract ideas (note: this wouldn't completely rule out abstraction, just the communication of it). If it's 2b, and they can communicate abstract concepts... that would be a monumentally profound discovery.