Clear Barrel Indicator, like a empty chamber indicator or chamber flag. It's just a length of weed wacker line long enough to go from the chamber end to the muzzle and stick out both ends indicating the barrel is unobstructed e.g. empty and 'safe'
Pretty strange just going off by itself. If there is any air in the tank at all, the valve is held against the seat and can't open unless the hammer hits the valve stem. I have modified valves in my Crosman guns that have much lighter springs than stock and I have never seen such a thing. More than once I have touched off a round with the breech open and the pellet does go somewhat down range but nothing else happens.