Is it illegal for a person to leave a vehicle unattended on private property?

Leaving a vehicle unattended on private property without explicit permission from the property owner is indeed generally considered illegal. This is because private property is owned by individuals or entities who have the right to control who can park there and under what circumstances. If a person parks their vehicle on such property without permission, it can be seen as a form of trespassing.

In many jurisdictions, property owners have the authority to enforce their parking rules and regulations, which may include having unauthorized vehicles towed at the owner's expense. Therefore, without the owner's consent, leaving a vehicle unattended not only violates the property rights of the owner but could also create liability issues for the vehicle owner if any damage occurs while it is parked there.

The other options do not align with the principles of property rights and vehicular regulations. For example, simply locking a vehicle does not grant permission for parking on someone else's property, and saying it’s always permitted undermines the rights of property owners to control access to their land.