Visible smell sensor? I'm a newbie to Blender and I'd be grateful if you could help me to solve my problem.

Imagine, there are multiple objects located in a 2D maze and all of these objects release different odors. Odors are spreading just like Wifi signals and they are visible, and our player can follow these odor signals to detect the location of these objects.

So how can I make objects to release such visual signals? And how can I make the player to detect that there is a signal exist in a specific direction.

Thank you very much in advance for your answers and please let me know if my problem isn't clear.
I would use a different colored smoke for each smell. You could use a mist effect and change the atmosphere color as you get closer to a smell, or you could use something like in this question How to create volumetric/smoke material in blender game engine?.