Ibn Ezra on the Torah

For he was a son of old age. As it sounds, for he begat him when he was 91. His brother Binyamin they also called "child of old age." And after these two none were born to him. passim, embroidered tunic. Passim is like pas yada (palm of the hand) in Aramaic.

The plain sense meaning is that one of the passers by found him.

When the Ishmaelite merchants passed by them, because the Midianites are called Ishmaelites. And so it says about the kings of Midian, that they were Ishmaelites (Shoftim 8:24).