"Jaques d'Alltrades" wrote in message

Sorry about the long reply but this is the actual law ............

Depriving someone of their property without legal right, but with no
intention of depriving them of it permanently (legal right might be for
instance, the police removing evidence) is a civil offence, and usually,
a criminal offence, too.

"Theft By Finding" is what it used to be called, I think; if you find
something that isn't yours and you keep it, then it's theft.

The info from various Citizens' Advice Bureau is:

If a child throws a ball into a neighbour's property the neighbour should
either hand it back or allow it to be collected. However, as it is a
trespass for the ball to cross the neighbour's boundary, even if it was
unintentional, the neighbour would be entitled to compensation if s/he can
prove s/he has lost money, for example, if the ball has smashed a window. It
is also theoretically possible that, if the child's ball is always coming
into a neighbour's property, the neighbour could take a court action for
nuisance with an injunction to prevent repetition. However, legal advice
would have to be sought. A conciliatory approach would be preferable.