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Everyone wears body armor except for Faye. Notice how her outfit looks a lot like a wetsuit? That's because it is a wetsuit; she mentions earlier that her team got ambushed; extrapolating from that, she lost most of her gear in the chaos, including her fatigues and body armor.
The American mechs have a more Western-style design (similar to Battletech), compared to the more anime-esque designs of the Japanese robots.
How do the Hollow Children grow up with a robotic endoskeleton? Fake memories are thrown out there once (possibly as a joke), but the first Hollow Child we see looks to be in his mid-forties or fifties and was apparently issued a green card thirty years previously.
There's every indication that Hollow Children are "built" as adults and given fake memories (the failed prototypes in the disposal pits strongly support this interpretation). This would also definitively rule out Dan being one.
How have the Hollow Children not been detected earlier? A simple x-ray or CT scan would pick up all the mechanical equipment inside, not to mention a metal detector. And none of them had a single severe accident prior to the first Hollow Child surfacing?
For that matter, how the hell did the Chairman of the Joint Chiefs pass all his yearly physicals without detection?
Anyone Beetle Team knows could be a Hollow Child. Anyone.Dan could be one.