[rincebrain]

There's all sorts of anecdotes of pets reacting to people being ill well before the people put 2 and 2 together.

The primary problem, I think, is that you'd have difficulty convincing someone to fund this research without a priori knowledge - e.g. if you don't already know that there are (relatively) readily externally visible biochemistry changes from a degenerative brain problem, why would you see if you can train an animal to smell it?

It's similar to the question people posed after the PS3 signing key leak came out - while it is the case that Sony was signing all PS3 (and PSP, if memory serves) binaries with the same (all-zeroes?) random input, so they leaked information sufficient to eventually retrieve the private key from enough samples, why would you think to check if they did that without already knowing?

(Or, more generally, the large domain of problems that is relatively-trivial to verify a correct solution but infeasible to test all possible solutions in order to find one.)