Here is the quote in question:
"Vilt said Kunz's parents left him with his grandfather the morning of his disappearance while they went to scout fishing locations."
(bbm) This reads as though Vilt is saying not that they left for the store in the morning but that they went exploring/looking for minnows/scouting fishing locations in the morning and that was when they left DeOrr Jr. with ggpa.
ymmv ... moo, imo.
That very same article states the mother saw the man staring at her son in the morning when they went to the store.