Trivia: The spears shown in the lab are from Predator 2 and Alien vs Predator.
Trivia: When Rory is walking down the street wearing the Predator mask, there is a bright red/orange alien walking toward him. This is a nod to the original Predator costume that looked like a big bug, the red/orange color was used to film the stealth camouflage scenes. The bug suit was scrapped after a few test shots proved how silly it looked. Jean-Claude Van Damme was in the original costume, but he was let go when the costume was scrapped. (00:45:20)
Trivia: Gary Busey, who played Peter Keyes in Predator 2, is the father of Jake Busey, who plays a doctor in this film.
Trivia: Spoilers. Originally, Shane Black wanted Arnold Schwarzenegger to make a cameo in the end - he would have been the "Predator Killer" that rises from the device right before the credits. When he decided not to return, the "Predator Killer" was changed into a highly advanced suit of armor.
Trivia: The line on the poster "You'll never see him coming" was used by The Mandarin from Iron Man 3, also directed by Shane Black.
Answer: Well, in the case of Predators, it was violent criminals, mercenaries and military being randomly kidnapped and dumped on an alien planet. From the perspective of humans, it would appear that the criminals and mercenaries had gone missing in a more mundane fashion or simply died. If the Predators were covert about it, the Predator researchers would not be suspicious at all. So it's entirely possible it's canon.