"hidden from common observation" means that if you'd bothered to look, and still couldn't see it, it's "hidden". That came up a lot for me about 20 years ago, when there was a statewide rash of carrying concealed without a permit cases. What the cops were doing is stopping motorists and making them get out of the car while they did a cursory search (what they could see from the windows using flashlights). If the motorist had a gun in a holster, openly, or a gun on the passenger seat, the cops would say that, since they were unable to see the gun when they approached the vehicle (right? not even close enough to actually look) that the gun was therefore "hidden from common observation". I always tried those cases (when the client didn't chicken out and take a plea) and in cross examining the cop I would ask questions about how diligent he'd been in looking (they had never bothered to look until after the driver had been detained - i.e., there had never been probable cause to detain the driver much less issue a criminal charge). I also made a big deal out of the fact that when they did bother to look, they had no trouble seeing that which had been open to public viewing all along. They stopped doing that as other attorneys picked up my techniques.