[kbenson]

I imagine it's probably that it's prohibitively expensive to make one with the qualities required, which is reliable, mobile, can alert others automatically, with a long running time.

Dogs (some breeds more than others) have some of the strongest smelling capabilities on the planet. So much so, that some people theorize that one of the reasons dogs don't traditionally do all that well on the mirror self-recognition test is that it focuses on vision, and as animals that rely so heavily on such a strong smelling capability, it may throw them off (I guess the human equivalent would be if the test was audio only, or audio with a very fuzzy image. If you weren't already familiar with hearing your voice played back to you, how likely would you be to recognize it as yours?)