To be a fugitive, there has to be something to be a fugitive from.  Somebody who is out "there" in the place where God is banishing Cain.  The only reason for a person to be afraid of being killed by "someone,"  is that there is "someone" out there who is trying to kill you.  

And God is in complete agreement with Cain that someone will try to kill him.  So much so, that God puts a mark on Cain.   Gen. 3: 15b,  "And the Lord set a mark on Cain, lest any (one) finding him should kill him."  God even set a curse on any person who killed Cain, "...vengeance will be taken on him sevenfold..." God said.

This scripture passage gives us two conclusive ways  of knowing for certain that there are people "out there":  1.  God gave Cain a mark lest anyone should kill him, and  2. God curses anyone out there  that kills Cain.  Whoever this someone "out there" is, they have the intelligence to recognize a mark that is placed on Cain by God, and intelligent enough to recognize what it means.

Further adding to the fact that there were people "out" there is validated in Gen. 3:16.  "And Cain went out from the presence of the Lord, and dwelled in the land of Nod, on the east of Eden.  And Cain knew his wife, and she conceived...".

Nod was a place with people living there.  Cain knew it.  And God knew it.  What does that mean?  We can look at the genealogy of Adam (and Cain), and determine how many years ago they lived within 5000 years or so.  We can't determine how long  people had been living in Nod.

But we can determine that Adam was unique in a critical way:  He was filled with the Spirit of God.
Adam and Eve were a new beginning of some sort.  A new kind of people.  Spirit filled.