Because your vehicle was towed the police may conduct an inventory search of your vehicle according an an inventory policy, if the police department has one. The inventory search is intended to protect you in case items of value are missing while the vehicle is in the tow company's custody. So the police shouldn't be reading your mail or searching for contraband. As to the expired registration, many courts will dismiss this if you show that you renewed.
My response does not create an attorney-client relationship nor should it be relied on as advice in this particular case. My response is provided for discussion purposes only. 413-528-1211.
Believe it or not, he was searching to proect you and the department--validating contents--an inventory search incident to seizure for the expired tag (that said, I am NOT a MA attorney and the laws in your state may be different than the general rule).
NOT LEGAL ADVICE. FOR EDUCATION AND INFORMATION ONLY. Mr. Rafter is licensed to practice in the Commonwealth of Virginia and the US Federal Courts in Virginia. His answers to any Avvo question are rooted in general legal principles--NOT your specific state laws. There is no implied or actual attorney-client relationship arising from this education exchange. You should speak with an attorney licensed in your state, to whom you have provided all the facts before you take steps that may impact your legal rights. Mr. Rafter is under no obligation to answer subsequent emails or phone calls related to this matter.