Much like the Great White Shark (Carcharodon carcharias), Megalodon was a warm-water shark, that lived in shallow seas. The Megalodon (if still alive) would have easily have been spotted by humans, especially now with all our boats and planes/helicopters. However, it would be impossibly for Megalodon to even survive in our modern oceans, as they are colder, there is less of a food supply, and there is a lot of competition for what little food there is. Also, that 10,000 year old tooth was found to be an inaccurate estimate, as scientists used a now-seen as inaccurate form of carbon-dating. However, our modern forms of carbon-dating will not work on the tooth, as it is not chemically possible with that specific specimen (Google it if you want more details).