In most US states, a person finding lost property, including money, must make reasonable attempts to find the owner, and restore the property, or else turn it over to the police so that they can search for the rightful owner. In some states it is required that the police or some other authority be notified. If, after a period defined by law (and which varies ...
In England, the situation you're describing would be classed as 'Theft by Finding' and would be an offence under the Theft Act (1968).
The fact that you found the money, seemingly abandoned or lost, would be no defence under law and is explicitly regarded as 'dishonest' appropriation.
if he appropriates the property in the belief that the ...
If it isn’t registered, it didn’t happen
That’s all there is.
The land registry in England and Wales is definitive for all transactions for the whole country since 1998 (the exact date depends on which part of the countries).
If the joint tenancy was registered and the change wasn’t then it’s still a joint tenancy - X owned the land on Y’s passing.
england-and-wales (things might be different in scotland and assuming the Enforcement Agents are in possession of a lawful writ or warrant)
The house sitter has to prove ownership
The law, at para 10, Schedule 12 of the Tribunals, Courts and Enforcement Act 2007 is that:
An enforcement agent may take control of goods only if they ...
This would be a violation of 18 USC 1030, which criminalizes knowingly accessing a computer without authorization. The provisions of this law are fairly open-ended. It depends on what exactly you do with the computer, for example it is a crime if you "obtain information" from the computer (read a file). There are state analogs to the federal law – ...