Here are some interesting human detection facts, from Uselessknowledge:
» You can see a candle flame from 50 Kilometers on a clear, dark night.
» You can hear the tick of a watch from 6 meters in very quiet conditions.
» You can taste one gram of salt in 500 liters of water (.0001M).
» You can detect one drop of perfume diffused throughout a three-room apartment.
» You can detect the wing of a bee falling on your cheek from a height of one centimeter.