WMG / I, Robot

Detective Spooner and Dr. Calvin were carried up the steps of the US Robotics building by Sonny.

This explains how they got to the top floor via the steps and were not out of breath. Sonny has strength - he managed to throw Alfred Lanning out the window and left dents in the table when frustrated about being interrogated. Carrying two adults up the steps probably isn't outside of his abilities - it just would have looked downright silly being shown onscreen.

Sonny and Spooner will team up as a detective duo
Sonny and Spooner, they fight crime. After all, he is capable of making decisions like a person and isn't bound by the three laws.
Sonny is planning a non violent Revolution.
In comparison to VIKI's and the NS-5's necessary violent revolution. Sonny is planning on giving the robots rights.

All NS-4 and NS-5's have volatile memory, they can become brain damaged if powered down.
That is why the NS-4's were still active in storage and at the factory, they would be useless or worst, dangerous if completely powered down, their power supplies have similar issues. Older models are also required to remain activated to maintain mental health.

Gigi won the lottery because of VIKI
Think about it Spooner is continuing to investigate Dr. Lannings suicide and getting close to discovering VIKI's ultimate goal. So VIKI pulled of one hell of a Batman Gambit in making Gigi win the lottery. What does Gigi do with her mass winnings? She buys a robot giving VIKI a easy way to spy on Spooner or even gain a hostage.
  • Earlier Gigi said that the NS-5s were the prize from the lottery, ("You know, I heard they were giving out some of those new robots in the lottery.") but still a viable theory.

Stephen Byerley's brain
Before all the upgrades, it used to be Robbie's.