My sister had a dog that wasn’t trained for it but was a great sniffer. I would throw a rock into the creek and she would go swim and bring back the same rock. When I first did it I thought it was so funny, thinking she was just a stupid dog and bringing me back any rock on the cobbly stream bed. It took me numerous throws until she brought back what appeared to be the SAME rock that I had thrown. So I made sure to pick my rocks out so I could remember them. Yep - every single time she would bring back the rock I had thrown!
It didn’t take me too long when I was out fishing in the boat with her to realize when I was in a good spot to fish - she would get antsy and excited and looking and sniffing at the water! After a day of fishing I figured it out, so afterwards I wouldn’t spend an hour fishing at a usual spot unless she acted up.
A few years ago my black labs would sniff my Ex’s knees whenever she visited. They were quite persistent and it annoyed her to the point that she Googled it. “You have cancer” was one of the top results. So she went to the doc for an exam and sure enough a lump was detected. Further testing indicated she had breast cancer. A year of treatment and she was cured. The dog’s have been on half rations ever since.