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However, neither the colonists nor any Company employees seem to know anything about the Derelict Ship until Ripley arrives back at Earth 20 years later (and the warning signal coming from the derelict ship was presumably deactivated or no longer operating after the Nostromo visited the planet). About thirty years after the events of the first film, the Company begins terraforming the planet and a colony is set up there. In that case, the Company probably had little other choice than to bury the project. Another reason could be that they simply had no opportunity: with the Nostromo declared missing, there were undoubtedly large inquiries done which would only have drawn attention to their unethical acquisition of the Alien. One reason may be that the Company's secret attempts to obtain the Alien were only known by a few of its executives (the ones who re-routed the Nostromo to LV-426's vicinity) and that they were no longer in a position to send another team to LV-426 after the events of the first film. This is not made completely clear in any of the films, though there are plausible explanations. In the novelization of the film (taken from the original shooting script), Ripley comments to Burke that the evidence had been tampered with, and later, when Burke is confronted with the extensive damage to the colony complex, he remarks that the complex is insured anyway. Ripley would be fully accountable for the destruction of the Nostromo, which would enable the Company to get their lost money back through insurance. This is further supported in that no mention was made of the fired harpoon gun, which was still in the shuttle when the salvage team found it. Since the discovery of some proteins or scorch marks is hardly conclusive evidence, it is very likely that the Company simply lied to Ripley and used it as a way to demote her and thereby silence her about the Alien (as they may have plans for it yet). However, since the Alien was bleeding when Ripley hit it with the harpoon, some drops of acid may have scorched the floor or wall, and because saliva also contains organic molecules, trace amounts of proteins should have been discovered if the team had done a thorough job. After being adrift in space for 57 years, any remaining drool would certainly have evaporated.
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The subsequent air displacement was strong enough to almost blow the Alien out (had it not held on), so it is imaginable that most of the drool blew out/evaporated quickly.
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Ripley's shuttle decompressed shortly after the Alien drooled on the floor.
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