I live in NYC. Notice that the city isn't banning hoax devices that *look* like geiger counters. They are only banning the real thing. This suggests they are more worried about the real thing getting into the hands of environmental activists than they are about preventing false terror alarms.

A few reports of a "funny smell" in the subway is major news even when machines can't detect anything. There have been huge outbreaks of a mysterious "maple sugar smell" [ http://gothamist.com/2007/11/16/as_seen_on_tv_t.php ], and other weird smells that machines can't pick up [ http://gothamist.com/2007/01/08/maple_syrup_was.php ]. A machine isn't needed to start a panic.