Failure to return thing lost, found = a crime (Theft)

Theft is committed by any person who, with intent to gain but without violence against or intimidation of persons nor force upon things, shall take personal property of another without the latter's consent. (Article 308 of the Revised Penal Code) Therefore, the elements of Theft are [a] taking of property of another; [b] intent to gain; and lack of consent of the owner or possessor.

However, theft is likewise committed by: [a] any person who, having found lost property, shall fail to deliver the same to the local authorities or to its owner; [b] any person who, after having maliciously damaged the property of another, shall remove or make use of the fruits or object of the damage caused by him; and [b] any person who shall enter an [enclosed] estate or a field where trespass is forbidden or which belongs to another and without the consent of its owner, shall hunt or fish upon the same or shall gather cereals, or other forest or farm products.

Therefore, if a person finds a movable (because immovable property cannot be lost), he should return it to the owner, if ownership appears, or, under Articles 719-720 of the New Civil Code, he should deposit it with the mayor of the city or municipality where the finding has taken place if the owner is unknown. Failing to any of the two actions required of him, the finder shall be liable criminally for Theft.

Below is a summary of the law on finders, keepers:

[1] Whoever finds a movable, which is not treasure, must return it to its previous possessor.
[2] If the possessor or owner is unknown, the finder shall immediately deposit it with the mayor of the city or municipality where the finding has taken place.[3] The finding shall be publicly announced by the mayor for two consecutive weeks in the way he deems best.
[4] If the movable cannot be kept without deterioration, or without expenses which considerably diminish its value, it shall be sold at public auction eight days after the publication.
[5] Six months from the publication having elapsed without the owner having appeared, the thing found, or its value, shall be awarded to the finder.
[6] The finder and the owner shall be obliged, as the case may be, to reimburse the expenses.
[7] If the owner should appear in time, he shall be obliged to pay, as a reward to the finder, one-tenth of the sum or of the price of the thing found.