(Exodus 22:3) says: "If found will be found in his hand." The phrase "in his hand" seems to mean the stolen object was physically held by the thief. But the Mekhilta interprets "in his hand" as meaning "in his possession" or "under his control" — not literally gripped in his fingers.
This interpretation is supported by parallel uses of the phrase throughout Scripture. (Numbers 21:26): "And he took all his land from his hand" — meaning from his domain, since land cannot be held in one's hand. (Genesis 24:10): "Then the servant took ten camels and all the good of his master in his hand" — meaning under his control, since no one physically holds ten camels in their hands.
From these parallels, the Mekhilta establishes that "in his hand" is an idiom meaning "in his possession" wherever it appears in the Torah. The stolen goods need not be physically on the thief's person when he is caught. They simply need to be under his control — in his house, in his warehouse, anywhere in his domain.
This ruling had major practical implications. A thief who stashed stolen goods in a hiding place rather than carrying them personally could still be convicted. The legal standard was possession, not physical contact. The Mekhilta prevented a thief from escaping liability by simply putting down the stolen object before being apprehended.