2,748. Declaring Property Temporarily Ownerless
Hilchos Nedarim 2:18
If a person declares property to be ownerless for a day, a week, a month, a year or a sabbatical period (i.e., seven years), he may retract before the property is acquired (even after the three-day limit described in the previous halacha). Once the property is acquired, either by its original owner or another, he may no longer retract. He may retract until it’s acquired because this is an unusual circumstance, given that a person doesn’t normally declare something ownerless temporarily.
Hilchos Nedarim 2:19
If someone comes and watches ownerless property, looking at it so that someone else doesn’t take it, he has not acquired it. Rather, he must pick it up if it’s movable property or demonstrate ownership of it if it’s real estate, the same way that buyers acquire property.