Would the group leader and the fighter who borrowed a car tire and were unable to return it to its original owner due to wartime circumstances and injury, and then could not find the owner afterwards, be sinful?

Whoever exerts effort in returning a trust and paying off a debt has cleared their conscience. An example of this is the story of the man who borrowed a thousand dinars and could not find a ship to return them. So he placed them in a piece of wood, threw it into the sea, and prayed to Allah to deliver them to their owner. Allah accepted his supplication.

If you search for the owner of the frame and cannot find him, then give its value in charity on his behalf, and the reward will be his on the Day of Judgment. If you find him later, then you are obligated to guarantee his right.