The ephod tells David Keilah would surrender him
Abiathar fled to David to Keilah, ephod in hand, and David used it immediately to confirm Saul was coming, and Keilah would surrender him over to him. (1 Samuel 23)
David wears an ephod to bring the ark into the city of David
When David brought the ark of the Covenant from Obed-edom's house to the city of David, he sacrificed an ox and a fattened animal when the ark movers had taken 6 steps. He danced before the LORD with all his might in his linen ephod, and all the house of Israel was shouting with the sound of the horn. Michal watched from her window and despised David in her heart, accusing him of uncovering himself in front of her servants' female servants, though David refused to be scolded for making merry before the LORD. (2 Samuel 6)
Israel uses ephods as part of their idolatry
And I said to her, "You must dwell as mine for many days. You shall not play the whore, or belong to another man; so will I also be to you." For the children of Israel shall dwell many days without king or prince, without sacrifice or pillar, without ephod or teraphim, Afterward the children of Israel shall return and seek the Lord their God, and David their king, and they shall come in fear to the Lord and to his goodness in the latter days. (Hosea 3:3-5)