That's interesting...thinking about ballons in history in wartime....
Probably nothing... but, a reminder
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http://www.ianlivingston.com/archived/2004/osamasrevenge_july14.htm
The CIA uncovered plans in one of Sultan Bashiruddin Mahmood's (Abdul Kadeer Khan's top associate) corporation offices:
"They also discovered illustrations running the length of the room that showed how high altitude balloons could be used to release large quantities of anthrax and bubonic plague bacilli over major metropolitan areas."
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Seeing that they have uncovered these plans, it seems weird that no fighter planes were sent to investigate what it was. I dunno.
The fugo balloon comes to mind.