What is the ruling on benefiting from a mobile phone memory card found in a purchased car, knowing that its original owner has not come to claim it despite repeated notifications?

It is not permissible to benefit from memory (storage device) whose owner is known except with his permission; otherwise, it is a trust. If you benefit from it without his permission, you have betrayed the trust, and you must guarantee it to him. Contact him and seek his permission. If he permits, then so be it. Otherwise, it remains a trust until he comes, unless you despair of his arrival, in which case it is permissible to benefit from it while guaranteeing it to him if he comes.