In the mission Till Death Do Us Part you will find a couple of men are fighting eachother in the front yard.

Now the egg is this small button on the left side of the house, on the ground, where the wall of the house meets with the ground.

The button is hard to find since it's so small and blends with the ground a little. But once you find it, shoot it. Doing so will cause the men who were fighting to come running over, stripped of their clothes as they applaud for you.