I've read of this before... it's absolutely extraordinary. My late poodle, Suzy had a tumor on her back and my sisters Jack Russell used to sniff and lick it all of the time. The craziest thing though, is that my brother-in-law Paul, has meniere's disease (it's an extreme case- you can check in out on the internet. The ears are affected by it) and he always licks his ears. We thought it was cute, but once I got Maximus and he started to mature, he now does the same thing every time he see's Paul. It's like they know something is wrong with his ears by the scent and lick them.
Whenever we have bruises or cuts, Maximus and Milo (JRT) sniff them out and lick them... however my sister's Pharoh Hound doesn't even notice. I guess it depends on the dog as well.