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I am sorry to hear about this situation. What happened was that the police towed the vehicle because its driver was arrested. At this point, the actual owner (i.e. yourself) can go to the police and request the vehicle/trailer back. They will release it, unless, the prosecutor requests it as part of evidence against your son for the criminal case (unlikely). If this happens (again, unlikely), then the vehicle and trailer would be released at conclusion of the case.
1) Yes, the trailer is considered part of the tow.
2) Items in the truck may be requested back and would be given back unless they can serve as evidence for the case.
3) By not receiving the notice, this does not invalidate the impound itself. However, this would indeed limit their ability to charge a full rate for the tow as they normally would.
4) You cannot file a lien on a trailer without the actual owner (i.e. the friend) owing money to you. Even if he did, the lien would not do much to invalidate the tow/impound or give you greater rights to retrieve it. If you want to retrieve it on his behalf, the easier thing to do would be for him to simply give you a Power of Attorney.
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