Independence Hall and the Liberty Bell, the Mall’s most famous occupants, have been closed to the public for the majority of the time since the shutdown began Dec. 22. That hasn’t stopped passersby -- including tourists catching a glimpse of the Liberty Bell from outside its windowed housing -- from leaving their cigarette butts, gum wrappers, and other items of trash behind, or the wind, for that matter, from blowing some of the city’s discards there.