In most cases, repossession agents are only authorized to repossess a vehicle if it is on public property or if they have permission from the owner of the property. If a repossession agent goes on someone else's property without permission, they could be liable for trespassing or other damages.
If you believe that a repossession agent has illegally repossessed your vehicle, you should contact the police and file a report. You may also want to contact a lawyer to discuss your legal rights and options.