FYI, it's a federal crime to mess with mailboxes, that's probably why they dusted for prints.
"Mailboxes are considered federal property, and federal law (Title 18, United States Code, Section 1705), makes it a crime to vandalize them (and to injure, deface or destroy any mail deposited in them). Violators can be fined up to $250,000 or imprisoned for up to three years for each act of vandalism.
You can take the following actions to help prevent the vandalizing or destruction of your mailbox and any mail that may be inside it:
Immediately report theft, tampering with, or destruction of mail and mailboxes to your local postmaster. You will be asked to complete PS Form 1510, Mail Loss and Rifling Report, or PS Form 2016, Mail Theft and Vandalism Complaint. These forms help the Postal Inspection Service determine whether your problem is isolated or one frequently experienced in your neighborhood.
Remove your mail from your mailbox as soon as possible after its delivery. Allowing mail to pile up for several days only tempts would-be vandals and thieves. If it is impossible for you to pick up your mail right after delivery, and you have been experiencing problems with mailbox vandalism or mail theft, consider renting a post office box, which will provide a secure method of delivery. You may find your peace of mind is worth the small rental fee.
Obtain Label 33 from the Postal Inspection Service. This sticker, which warns that willful damage to mail boxes and theft of mail is a crime, can be affixed to your mailbox.
If you discover someone tampering with your or your neighbor's mailbox, obtain that person's description and that of his vehicle, including the license plate number. Immediately report the information to your local police and postmaster.
Keep your mailbox in good repair by using proper installation methods. This may help prevent theft of the mailbox itself."