What caches must have in common is the scent of many different humans (trying to think like a dog here). I often feel deprived because my dogs are clearly enjoying an entire world of smells that I have no concept of. I really wish they could tell me who has been along this trail, who went off the trial, what other dogs have been along here, - there is whole world that they know but we don't. If dogs can be trained to find drugs they can be trained to find geocaches - we just need a reliable method. My suggestion is the smell of "other" people.
Thanks for your reply!