AIt's interesting to note, that in the AvP canon, the predators all have acid-proof armor and weapons. However in this film, when Celtic Predator was fighting the Alien, his wrist blade melts away after using it on an alien, then he gets some acid on his armor and it starts to melt. However, Scar Predator used his wrist-blades and many other weapons on the Aliens and they didn't melt at all. (We never see Gill/Chopper Predator use his gear because he was killed too quickly). This could have partly meant why the ritual went awry so fast is that the team leader (Celtic) was not properly equipped for the hunt, he may have grabbed the gear for hunting humans instead of the gear for hunting aliens, or there was a supply mix up. (this would explain why he paused to look at what was left of his wrist-blades and seemed surprised that they had melted) It is not uncommon for Earth militaries to accidentally send the wrong, or defective, equipment into a battlefield, so what's to say the Predators' Warrant Officers and other supply personnel don't make mistakes? It's possible that Scar was the only one who knew about the acidic blood; he might have been the only one who actually did his homework and researched the Aliens, either forgetting or choosing not to pass the relavent information on to the other two. (Equivalent to a group of College/University room-mates where only one actually studies!)
AThey didn't bother to check. They knew he was dead, so they had no reason to believe he had a living Alien embryo inside. Also, the Predators weren't wearing masks, e.g., Scar had to alter his vision mode to see the embryo inside Sebastian. Or they simply forgot at first as they were busy executing their mourning rituals; before someone could think of checking for chestbursters, the hybrid Alien had already come out and matured. That said though, considering that the Predators are fully aware of the Xenomorph lifecylce and the danger they pose if control was lost, it would make sense that they would take every precaution possible, even if it involves examining a Predator who happened to die in some other way (like Scar). In the extended version of Alien 3 it is shown that the host does not necessarily need to be alive while the embryo is developing and ready to be born, so even though Scar was killed by the Queen, the embryo inside did not die with him and emerged from his body later on. The very fact that these trainees were sent into a place infested with them and would be dealing with them directly is reason enough to consider any possible way one of the creatures could slip past them regardless if any of them were to actually make it out alive.For intelligent beings that possess technology we have only ever dreamed of and go far beyond our current engineering capabilities (interstellar travel, active camo, energy weapons, etc.), it is rather ironic and somewhat humorous to see that they would fail to consider the logic behind such things under the given circumstances. For example, you don't have people studying or working with dangerous viruses (figuratively speaking here) who simply walk out of a room when they are finished whatever they were doing without first undergoing some kind of decontamination process, even if there was no "accident" and everyone seems fine. In a sense the Xenomorphs can be viewed as the "virus" and the Predator's lack of a thorough post-examination on both dead and live subjects is just asking for trouble, especially if a live subject does not know or remember being impregnated. Scar for instance might not have been aware of what happened to him while he was marking himself, similar to how Kane in Alien had trouble recalling what happened when he was attacked. The important difference between the two here is that Scar knew the lifecycle and Kane did not. Scar's case is interesting because despite his aforementioned knowledge, suddenly getting knocked unconscious and waking up later on the floor, he doesn't seem to be able to connect the dots. After regaining consciousness he should have immediately (or at least at some point) become suspicious of the nature of the ambush and question why he had not been killed when he was clearly vulnerable. Also, had Scar known he was now a Xenomorph host, it is safe to assume that he would have not tried to escape the pyramid at all, it is more likely that he would have made use of his self-destruct device instead. In short, they (Scar included) SHOULD have checked, as it would have saved the Predators a hell of alot of trouble in the long run.Then again, we don't know how Predators have obtained their current level of technology; they may have simply stole it from another nearby and more intelligent species, which they could conquer with their normal technology; or maybe there are intelligent Predator scientists, but the technology is primarily used by Warrior Predators, who lack the intelligence and discipline to use it ethically and safely. The fact that AvP: Requiem showed us a Predator seemingly specialized in 'cleaning up' such mistakes created due to the sloppiness of others, may be an indication that such mishaps occur much more often. As far as the Sebastion thing goes, there seems to be a small amount of time in between when a person is captured and brought to the hive and when the egg actually opens up and the facehugger slowly begins to climb out (like Newt in Aliens), so Scar was probably scanning him to see if he had already been facehugged and impregnated or if he was still in the process of waiting for this all to happen.
AIt was confirmed by writer/director Paul W.S. Anderson that Schwarzenegger was going to reprise his role as Dutch and appear at the end of the film if he lost the California election. Since he won, however, he turned down the role and was written out of the script. It is unknown how he was going to make his appearance. One possible approach would have been to appear at the whaling station to investigate the loss of communication with the Weyland team and the explosion at the whaling station. Since he was the leader of a rescue team in Predator, he would have probably arrived at the whaling station and found Lex after her meeting with the Predator tribe and taken her back to the boat. Originally in Predator 2, the role of Peter Keyes was actually supposed to be Dutch Schaeffer who is now part of an elite unit and whose specific duty was to track and capture the Predator. This was changed when Arnold turned down the role because he didn't like the idea of taking the Predator into the city. His cameo might have re-envisioned this role.
