705.102 Reporting lost or abandoned property.— (1) Whenever any person finds any lost or abandoned property, such person shall report the description and location of the property to a law enforcement officer. ... (3) It is unlawful for any person who finds any lost or abandoned property to appropriate the same to his or her own use or to refuse to deliver the same when required. (4) Any person who unlawfully appropriates such lost or abandoned property to his or her own use or refuses to deliver such property when required commits theft as defined in s. 812.014, punishable as provided in s. 775.082, s. 775.083, or s. 775.084.*
Similar laws are on the books in many US jurisdictions. It is actually quite common to bring police into these matters. (P.S. You might reasonably think of e.g. an envelope with one paycheck worth of money as the upper bound on property likely to be found abandoned, but a lot of the legal cases for this sort of thing involve cash hordes or doing things like abandoning e.g. a building or very valuable capital machinery.)
There's a locally famous case in my neck of Japan where a construction company, hired to break ground on a new building, discovered approximately $300,000 in cash buried in a tin can. It actually went before a magistrate, quite unusually for Japan, because it was very unclear who owned the money. The ruling was that the construction company received 25% (for having legal control of the work site) and the workers received 75% (for having actually found the money), while the land owner received 0%, because they had "no cognizable interest in property which they were unaware existed", and the (unknown) owner of the property had forfeited any interest in it by abandoning it in circumstances where they could not reasonably have precluded others from accessing it.
The court might have been smiling as it said that, particularly to the land owner, because the "Everybody knows it and nobody is saying it" conclusion was that the only reason somebody buries $300k and refuses to acknowledge it when it is unearthed is because the money is dirty. (The rumor was "untaxed proceeds of mob operations.")