It doesn't say he disappeared, Kenny just said "where the f*ck is luke" doesn't mean he disappeared he might be getting supplies and Kenny could be getting impatient
He is a ghost or he is a coward, in this context those are both mutually exclusive. He isn't a ghost because that would be stupid and in episode 3 it is clear that he is the furthest thing from a coward.
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*PLOT TWIST*
He may have died doing something important for all we know.