A lot of things that used to be science fiction sounded crazy to people before the physics was well-understood...ultimately, the brain is an electrical system and at a certain degree of sensitivity the signal emanating from its functions can be archived and indexed...

We have satellites capable of looking hundreds of millions of lightyears in distance through our galaxy, so it isn't all that hard to do a little bit of math to figure out how much sensitivity would be needed to read brain signatures...

Anything which uses energy emanates heat, so it's really only a matter of triangulating location and canceling out irrelevant signals nearby...

There is a common misconception that technology which interacts with the mind is categorically unsound, but before you dismiss it outright take a look at the physics behind it.