This easter egg is located at the back of the house where the guards are doing push-ups. If you have already got to mastery level 18 or more then you can save yourself some time by starting as a hacker in the house.
1. Disguise as a hacker or Militia Elite,
2. Go to the back of the house where you find the 3 guards doing push-ups.
3. Pick up an apricot ( located near radio or right behind the officer in charge of the 3 guards ).
4. Throw the apricot ( from behind is best ) at the commanding officer giving the instructions.
5. Run to the radio beside the guards, turn it on and they'll start dancing.
Challenge/achievement unlocked.
Video explaining below.