Final answer:
The clues that Greta and the other campers were not supposed to be out at night include protective actions taken by characters, descriptions of the evening atmosphere indicating danger, and the behavior of campers avoiding nighttime socialization.
Step-by-step explanation:
The evidence that supports the conclusion that Greta and the other campers are not supposed to be out at night can be inferred from several excerpts in the provided text. For instance, the passage where Venters goes softly to look at the girl and covers her with a blanket before returning to the camp-fire suggests a protective action consistent with ensuring someone is safely inside at night. Furthermore, the description of the evening as the "chill of the evening was in his limbs, the fear of the gloom in his heart" as the little sleeper rose to his feet conveys a sense of danger or apprehension associated with the nighttime.
Additionally, the aforementioned sense of danger is reinforced by phrases such as "... it was exceedingly sultry. Not a breath of air stirred..." which indicates a stagnant atmosphere, commonly felt during the night when the air tends to become still. The fact that campers had avoided socializing outside their groups and a mention of everyone being at a special plastic fair further underscores that night time was not meant for venturing out.