Final answer:
A ransom note and clues from a victim's positioning, like Blake clutching a gun and the finding of a set of buttons under a skeleton's collarbone, are two potential clues indicating the presence of a killer at a crime scene.
Step-by-step explanation:
Two clues that the killer left behind at the crime scene in the passages provided include detailed observations and hints to the murderer's presence. From the descriptions, one clue is the delivery of a ransom note which displays a potential motive and timing of the crime. The typewritten nature of the note could provide forensic evidence. Another clue is the positioning and state of the victims, such as the corpse of the child and the old salt, or Blake laying on the floor clutching a gun, which might indicate a struggle or the cause of death. In addition, the presence of personal items and their placement, such as the set of buttons under a skeleton's collarbone, suggest that the victims were clothed at the time of burial, thus adding another layer to understanding the crime.