And they did. We saw them. Maybe they could have been "recalibrated" to detect the motion of the hamsters, but I don't see how that would have helped them detect a motionless egg.
Ah, my apologies. I was not referring to the motion trackers carried by the marines. I was referring to the onboard systems, ships AI, whatever they may have had. If such a system was incapable of locating and warning the crew of an unknown biological presence, then given the information obtained from Ripley before arrival, surely something could've been set to search for signs of non-human biological material. Motion aside. Could there not exist any technology that could identify and locate such a thing? Especially given the fact that they're familiar with "bugs". Surely the military of the future can detect the presence of biological weapons or unknown materials onboard their ships?