Short answer. If you discover the item was stolen and you know who the true owner is, you must notify that owner of the deposit. If the owner does not claim it within one month despite being informed, you are relieved of responsibility by returning the item to the person who deposited it.
What the law says
should he discover that the thing has been stolen and who its true owner is, he must advise the latter of the deposit
Civil Code, Article 1984 — No Duty to Prove Ownership; Stolen Things. Read the full provision →
No duty to verify ownership at the outset
Article 1984 starts with a rule that protects depositaries: the depositary cannot demand that the depositor prove his ownership of the thing deposited. When someone hands you an item for safekeeping, you are not required to investigate its origin or ask for proof of title. You are a custodian, not an authentication service. This baseline rule means you accepted the item in good faith and are not immediately in the wrong simply because the item turns out to have been stolen.
What the law requires once you find out
The situation changes once you actually discover two facts: that the item was stolen, and who the true owner is. When both are known to you, the law imposes an active duty: you must advise the true owner that the item is in your possession. This notification duty is mandatory — the word "must" in the article leaves no discretion. You are not required to hand the item over immediately, but you cannot sit on the information and continue treating the depositor as the rightful party.
The one-month window and what follows
After you notify the true owner, the owner has one month to claim the item. If the owner, in spite of such information, does not claim it within the period of one month, the depositary shall be relieved of all responsibility by returning the thing deposited to the depositor. That one-month period is the law's way of giving the real owner a meaningful chance to act. If the owner lets it pass without claiming the item, the law no longer holds you responsible for what happens when you return it to the depositor. Your obligation is discharged.
Suspicion without certainty — a separate rule
The law also addresses a situation short of certainty: if you have reasonable grounds to believe that the item was not lawfully acquired by the depositor — but you cannot identify a true owner — you may return the item to the depositor. This is a permissive rule, not a mandatory one. It gives you an exit when you are uncomfortable continuing to hold the item but the true owner is unknown. Whether to act on this option, and how, depends on the specific circumstances.
Protecting yourself in this situation
If you are in this position now, document every step. Note when and how you learned the item might be stolen, who you notified, and what response you received. If the true owner has been notified in writing, keep that record. Do not simply hand the item to whoever shows up claiming to be the owner without some reasonable basis to believe the claim — the law requires you to advise the owner, but prudence still applies. If you are uncertain whether you know enough to trigger the notification duty, or worried about liability from either side, getting legal advice early is the safest path.