Some years back the shop I dealt with had an incident where a customer was buying a hard, 2 gun case. The sales/display area was long & narrow so the merchadise was scattered over quite a long strip. Anyway, the customer brings the case to the register, price sticker up so the clerk can read it without moving the case. Call it premonition or suspicion or whatever but the clerk picked up the case to slide it onto the counter and it felt like something was inside. Opening it, he found a M700 Remington and a double barrel shotgun, both removed from the used gun rack. The customer naturally had no idea how those guns got inside the case and couldn't tell by the weight that there were 2 guns inside, Yeah, right.
I'm not trying to defend the dealer in the DC sniper case, just showing that guns can disappear from a shop and many times there is no explanation for the disappearance. If my friend hadn't checked that gun case he would never have been able to explain where those two guns went.
