In the absence of law or contract, abandoned property can be considered a gift under most circumstances, when the owner is not obvious, the property is delivered in a lawful manner, and there is no fraud on your part. There are exceptions, for example anything mailed to your home absent your request is considered a gift (in the US).
Car boots are covered under law, describing their use and ownership. They are not considered abandoned when used within the confines of law.
Vehicles and other property that typically has a registered owner must go through a legal process or registration for ownership to change (in general).
When damaging property that is not your own, you can be sued in civil court where guilt is assessed based on a preponderance of the evidence.