A 9 volt battery is a fire hazard because the positive and negative posts are on top right next to one another.
If this comes in contact with anything metal (aluminum foil,brillo,etc...) it will spark, and if there is a fuel for this spark you will have a fire. (fire needs heat,fuel,and oxygen to burn) To test this theory put a 9 volt battery in your pocket with some change,(use commonsense) this will bring on a whole new meaning to the words, "Hot Pants."
When you dispose of this type of battery (positive and negative on top) Make sure it is safely wrapped in electrical tape or something to keep it separated from anything else that may come in contact with it.
A small box or Ziploc bag if in a junk draw I have seen in some stores now that the manufacturers are now packing them with plastic caps. If you need to purchase a 9 volt battery try to find those that are packaged in this manner.