It wouldn't be that hard. The creature would have to live in a place where people already die or go missing in large numbers on a daily basis.
Real Earth is full of such places.
The Syrian Civil War alone has been going on for some five years and has a death toll estimated at 470,000. That's about 94,000 violent deaths per year, with bodies likely to go missing, in a country about as big as Washington. No one is going to miss one random people disappearing every other day in such places. Blame it on the war.
In a fictional scenario, a human hunting creature could survive in zones of war. They might have to do a lot of roaming. Superhuman powers that help surviving shootings and bombings would be a plus.
I live in Brazil, and we have a huge problem about kids going missing. Forty thousand children go missing every year. Most are never found. Granted, Brazil is a very large country. But I did some research in English sites... I not only found the same number I usually get from our own newspapers, I also found data for other countries. In the United Kingdom, one child goes missing every five minutes.
Missing children would quickly cause a lot of problems for the hunter on small towns, or rural places. But not in large metropolitan areas, specially if they are full of slums. Missing children are practically an epidemic in cities like São Paulo and Rio de Janeiro, and this is a problem far from solved. The predator of humans in this case would benefit from stealth or social skills.
The predators could even be cannibalistic humans in this case. This is actually starting to give me the creeps, so let's move on to another scenario.
A variation from the previous theme. UN estimates have two and a half million people being victims of human trafficking in 2008.
In a fictional overpopulated world, with many corrupt countries and weak borders, a predator could prey on people that are being moved around in this way. Victims of human traffick usually disappear to never be seen again.
There are two possibilities: either the human eaters are customers of the traffickers, or they fight the traffickers for the prey.
If the first case seems unlikely, remember that just a few centuries back taking people from Africa and selling them as slaves in the New World was a lucrative business. A lot of the would-be slaves never made it through the trip. It can be said that humans have historically been kidnapping other humans and taking them on a trip to certain death without a second thought.
In the second case, the man-eaters might even be a known risk involved in taking people across dangerous borders. Sadly, if the hunters limit themselves to picking on people whom most people and governments care little about, their existence could be common knowledge, and they would still be relatively safe from any organized human effort to eradicate them.
I imagined the three scenarios above having in mind a predator that eats the flesh of humans the way a large predator (such as a tiger or a lion) would. Cannibalistic humans would even fit the second and third.
In those scenarios, the predator has to live among humans, so they have to hide their presence in one way or another. They also have to be careful about how they handle the remains of their victims.
What about a super predator that doesn't need to care about hiding themselves nor careful corpse disposal?
- They came on a flying saucer
Set up an idyllic environment where people would like to go camping. Then wait until humans come. Use some bright lights for effect, as this may both mesmerize and lure them towards you. Then pull them into your space ship with your tractor beam and leave Earth. Remember, in space no one can hear them scream.
At some point people are bound to notice a lot of them will go missing. But the ones who escape the fate of becoming hamburgers for greys will be labelled as crackpots by the rest of society.
All the predator needs to be safe in this case is a huge compendium of literary production - TV series, book novels, videogames, movies - selling them as fictional creatures.
Last but not least, one of my favorite videogames has a variation of sorts on this theme. In the X-Com series of videogames, humans are always defending Earth from alien invasions, and the older games of the series had the humans serving as food for the alien at some points.
Plot twist: In X-Com: Apocalypse, a group of humans form the Cult of Sirius, a religious organization that believes in redemption by helping the same aliens that want to eat them. From the game wiki:
Official Entry: " This bizarre cult has whipped up a religious frenzy following the appearance of the Dimensional Gates. The cult has long believed in redemption of the human race by a superior Alien race. They believe the UFOs and Aliens to be harmless and are rapidly gaining credibility and recruits from the general populations. This represents a considerable threat to X-COM because the cultists will do anything to assist the Aliens in their purpose, whatever that might be."
It may be worth noticing that the world in question is post-apocalyptic (not surprising, given the name of the game). While many people have an utopic life, most are living in some kind or another of cyberpunk slums. Aliens use the bait of "redemption" to give these people some flicker of hope for a better life, even if it's an afterlife. And this is enough for the people to sacrifice themselves willingly for the aliens' appetite.