finding something in California, knowing who it belongs to (which the original "finder" knew) and not returning it, constitutes theft
That doesn't seem to be clear, and may possibly be flat-out wrong.
California Code states that a finder must contact the owner, if known, in a reasonable amount of time.
Any person who finds a thing lost is not bound to take charge of it, unless the person is otherwise required to do so by contract or law, but when the person does take charge of it he or she is thenceforward a depositary for the owner, with the rights and obligations of a depositary for hire. Any person or any public or private entity that finds and takes possession of any … personal property, … shall, within a reasonable time, inform the owner, if known, and make restitution without compensation, except a reasonable charge for saving and taking care of the property.
If the owner is unknown or doesn't respond, and the item has a value over $100, it is to be turned over to the police or sheriff, and if they don't hear anything within 90 days, the ownership of the item goes to the finder.
2080.1.- If the owner is unknown or has not claimed the property, the person saving or finding the property shall, if the property is of the value of one hundred dollars ($100) or more, within a reasonable time turn the property over to the police department … or to the sheriff's department ….
- The police department or the sheriff's department shall notify the owner, if his or her identity is reasonably ascertainable, that it possesses the property and where it may be claimed. …
2080.2.If the owner appears within 90 days, after receipt of the property by the police department or sheriff's department, proves his ownership of the property, and pays all reasonable charges, the police department or sheriff's department shall restore the property to him.
2080.3.- If the reported value of the property is two hundred fifty dollars ($250) or more and no owner appears and proves his or her ownership of the property within 90 days, the police department or sheriff's department shall cause notice of the property to be published at least once in a newspaper of general circulation. If, after seven days following the first publication of the notice, no owner appears … the title shall vest in the person who found or saved the property ….
(I elided irrelevant details.)
Also, the duties for a "depositary for hire" state that the depositary has no obligation to return an item without demand from the owner:
A depositary is not bound to deliver a thing deposited without demand
I cannot immediately find any criminal liability for a "depositary for hire" who fails to return an item, or for failing to hand over an expensive item to the police, which seems to be the only possible wrongdoing that happened here.