It's hard, because like we as human can distinguish each single colour from our high resolution screens, dogs can distinguish smell at such levels. read more
For dogs that work in the field, such as some hunting dogs and herding dogs, tails can collect burrs and foxtails, causing pain and infection and, due to the tail's wagging, may be subject to abrasion or other injury while moving through dense brush or thickets. read more
There may be sniffer dogs checking luggage, but, as others have said, that rarely happens in view of passengers except in customs areas. You’re pretty likely to encounter a very well-behaved dog (usually a beagle in my experience) on a leash in a customs area. read more
Hence why they spend so much time snuffling close to the ground. Therefore the higher up the smell is, the less likely the dog will locate it. Unless the handler is willing to lift the dog up so it can hoover everything above shoulder height, the dogs search is restricted to whatever locations it can clamber into. read more