Answer
(Fatwa: 1010/1010/M=1432)
In the term of Shariah, such thing is called ‘luqta’. It is a trust with you. Having it in your possession, keep on announcing and propagating about it on different places and search for its owner as much as possible. When you are hopeless that you will not get its owner then give it is charity to any poor on behalf of its owner.
وعرف اى نادى عليها حيث وجدها وفي المجامع إلى أن علم أن صاحبها لا يطلبها …. فينتفع الرافع بها لو فقيراً وإلا تصدق بها على فقير الخ (شامى زكريا 6/435)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband