Short answer. Finding it does not make it yours. The law treats a movable as still possessed by its owner even while he has no idea where it is, so what you are holding still belongs to someone. Your duty is to return it, or to turn it over to the authorities if you cannot identify the owner.
What the law says
The possession of movables is not deemed lost so long as they remain under the control of the possessor, even though for the time being he may not know their whereabouts.
Civil Code, Article 556 — Possession Of Misplaced Movables. Read the full provision →
"Lost" in ordinary speech is not "lost" in law
The starting point is a rule that sounds abstract and is in fact the whole answer: The possession of movables is not deemed lost so long as they remain under the control of the possessor, even though for the time being he may not know their whereabouts. A wallet that slipped behind a car seat, a ring dropped in a hotel room, tools left in a rented unit — the owner does not stop possessing them merely because he cannot say where they are. Legal possession survives the owner's confusion. That is why the person who picks the item up is not a first possessor taking something ownerless; he is a holder of another person's property.
So what you must do with it
Because the thing is still someone's, the finder's obligation is to get it back to that person. If you know or can readily work out who lost it — a name on a card, a phone that rings, a receipt inside the bag — return it to them, and it is worth keeping some record that you did. If you cannot identify the owner, the Civil Code does not leave you to guess: the finder is required to turn the item over to the local authorities of the place where it was found, so that the loss can be publicised and the owner given a chance to claim. Quietly keeping it, selling it or pawning it is not one of the options the law offers.
What the finder can get
Handing the item in is not a thankless act. Philippine law does provide for the finder: an owner who reclaims the property is expected to reward the finder, and where the prescribed waiting period passes with nobody claiming the thing, the law allows it to go to the finder rather than to the State. The point of the procedure is that these rights come through the process, not around it. A finder who followed the rules ends up with a clean claim to the item or to a reward; a finder who kept quiet ends up holding property he cannot explain, and explaining it later is a great deal harder than reporting it now.
Where this can turn serious
Two situations deserve caution. Property found inside a workplace, a vehicle, a rented room or an establishment usually has an obvious channel — the employer, the operator, the front desk — and using that channel protects you. And keeping a found item, once you know or suspect who owns it, can be treated as more than a civil matter; misappropriating property that came into your hands lawfully is dealt with severely, and value affects how severely. If the item is valuable, if someone is already accusing you, or if you have held it for a while and are unsure where you stand, take advice before deciding what to do with it.