Justice Sotomayor filed a concurrence which can be found in the Orders List.
This is not a Second Amendment case but it is remarkable for two reasons. It is the rare pro se case in which the Court requested a response and because it has to do with “mens rea or scienter” which requires that one know that what he did was a crime for him to be convicted of a crime. Although there are some conflicts in the California courts they have for the most part held that the crime of carrying a weapon illegally is a crime regardless of whether or not the weapon was carried without a criminal intent. The same is true with possession of a firearm in California. If one has an “assault rifle” as defined under California law but does not know that it is an assault rifle and it is not registered or otherwise legally possessed then one is guilty of the “crime” of possessing an unregistered assault rifle.
“”The policy underlying the prohibition against concealed weapons is based on the protection of those persons who may come into contact with a weapon bearer. If a weapon is not concealed, one may take notice of the weapon and its owner and govern oneself accordingly, but no such opportunity for cautious behavior or self-preservation exists for one encountering the bearer of a concealed weapon. In light of this policy, the question whether a particular weapon was concealed should be considered from the point of view of one approaching the location of the weapon, and the intent of the defendant as to concealment should not be considered, since a defendant’s innocent intent does not make a concealed weapon any more visible.” People v. Mitchell, 209 Cal. App. 4th 1364 – Cal: Court of Appeal, 4th Appellate Dist., 1st Div. (2012) at 1371. (emphasis added)
Here is a link to the case at SCOTUSblog.