We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who answered as follows:
He should give it in charity.
Question:
Is this Luqtat al-Haraam (lost property in the Haraam)?
Answer:
It belongs to a specific person but he is not known, so he should give it in charity on his behalf. It is not like Luqtat al-Haraam, which refers to property whose owner is not known. And Allaah knows best.