How can a manufacturer verify a circuit design?
Integrated circuit designs are done in teams because it is a very complex task. It would be very difficult for a single design engineer to hide something. A number of people would have to work on such a conspiracy.

From a designer's point of view an RF block is something completely different than a digital block. It looks completely different in the chip layout and would stick out like a sore thumb during a closer inspection of the fabricated chip. Not to mention the fact that additional circuitry would require additional die area and it would be very difficult to add something to a chip that had already been brought into a nice rectangular shape.

During the manufacturing process failures can happen and it actually has to be expected that the chip that comes out of the fab does not have to intended functionality. For this reason chips are tested on die level as well as on package level. Some of the methods (-> google) are Iddq testing, ATPG and BIST.
If some irregularities are discovered a broad range of failure analysis methods is available for further analysis.

Chips that emit RF could also be discovered during compliance tests (FCC/CISPR) where emissions are measured in an anechoic chamber. Sometimes chips are the source of EMI problems and closer inspection using for example surface scan can be used to locate the area on a chip that generates EMI.

Finally, I think that almost everything is possible, however in this case it would be a very difficult endeavor.