I opened my shed one day and was greeted by the unmistakeable odour of a dead animal.
I had no idea what had conked out, but worse was I didn't know where it might be; it's a big shed and cluttered.
Milly (the dog) however had no such problem, walking in and going straight to the source. A dead rat in one of the framing steels.
To a dog there seems to be no such thing as a good smell or a bad smell, just a smell. And with a nose thousands of times better than our own at not only detecting smells, but finding the source it seems they just use the odour as information. Unlike ourselves who tie up smells with emotions, the dog appears to use the information in a purely intellectual way.