Albert Barnes Commentary 1 Samuel 21:9

"And the priest said, The sword of Goliath the Philistine, whom thou slewest in the vale of Elah, behold, it is here wrapped in a cloth behind the ephod: if thou wilt take that, take it; for there is no other save that here. And David said, There is none like that; give it me." — 1 Samuel 21:9 (ASV)

Wrapped in a cloth behind the ephod — This should be translated as "in the cloak," referring to Goliath’s military cloak, which was part of the dedicated trophy. The ephod was naturally hung up where the high priest alone could access it.