I like the possibility that Ardyn’s the one who smuggled Prompto out of the research facility, because it was either:
a)“Ah, step one in sub-part gamma of my painstaking arrangement of events to fulfill the conditions of the prophecy, subtly manipulating conditions in order to get this boy to Insomnia so that the Chosen King may have his ordained companions, and thus move myself one inch closer to my ultimate goal.”
or
b) “You know what I could do today? Steal a baby.”