What about the story of Balaam? The angel of the Lord came to Balaam in a spirit form and only the donkey could see him. How else could the angel of the Lord be present but invisible? What else besides spirits can be present but invisible?
But Gods anger was kindled because he went, and the angel of the Lord took his stand in the way as his adversary. Now he was riding on the donkey, and his two servants were with him.
23And the donkey saw the angel of the Lord standing in the road, with a drawn sword in his hand. And the donkey turned aside out of the road and went into the field. And Balaam struck the donkey, to turn her into the road.
24Then the angel of the Lord stood in a narrow path between the vineyards, with a wall on either side.
25And when the donkey saw the angel of the Lord, she pushed against the wall and pressed Balaams foot against the wall. So he struck her again.
26Then the angel of the Lord went ahead and stood in a narrow place, where there was no way to turn either to the right or to the left.
27When the donkey saw the angel of the Lord, she lay down under Balaam. And Balaams anger was kindled, and he struck the donkey with his staff.
-Numbers 22