The answer to this question is odors traveling long distances from their source.
Gizmodo.com's article What is the speed of smell? notes that smells are carried by odorous gas molecules through the air. Knowing this, we can at least take a stab at working out the speed of smell.
At the most basic level, particles tend to move from areas of high concentration to areas of low concentration until equilibrium is reached - this is known as diffusion.
According to Kinetic Molecular Theory, gaseous particles are in a constant state of motion, moving at random speeds and in many different directions. Because of their kinetic energy at temperatures above absolute zero, all particles undergo diffusion.
Diffusion refers to the process of particles moving from an area of high concentration to one of low concentration. The rate of this movement is a function of temperature, viscosity of the medium, and the size (mass) of the particles. Diffusion results in the gradual mixing of materials, and eventually, it forms a homogeneous mixture.
See Boundless.com's Gas Diffusion and Effusion and Okstate.edu's Gas Laws to learn more.