The town of Acton, just west of Concord, has Isaac Davis's plow, and bloody shirt. Isaac Davis was captain of the Acton Minutemen, and died in the first volley at the Old North Bridge. It is his plow that is depicted in the Daniel Chester French Minuteman Statue by the Old North Bridge. The face is his great nephew, who was said to be his 'spit and image'.
The Old North Bridge is famous for being rude, but it didn't give me any trouble.
Most historical sites run by Federal, State, City, etc., do not allow metal detecting, but that doesn't mean people can't go there at night and do it, although it would be hard to see what you're digging without a flashlight, which would likely give you away anyway. I know there have been instances in the past of people metal detecting at Gettysburg Battlefield sites. I'm sure it goes on in plenty other historical sites.