Plot hole: As the evolved Predator's ship takes off it leaves Casey Bracket alone in hilly forest terrain. The ship then flies a long distance before crashing and the Predator and McKenna start fighting, just for Bracket to show up and help out of nowhere. Impossible for her to cover that distance that fast in hilly forest terrain.
Plot hole: Quinn gave the box containing the Predator's equipment to a Mexican and told him to send it to a P.O. Box in the U.S. He also gave the man an unknown amount of money for the postage. Without questioning IF the Mexican would follow his instructions and had enough money, sending a package internationally is not that simple. The package would need to go through customs and several documents would need to be completed. The contents must be disclosed. It is doubtful it would pass Customs.
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.