To reconnoiter is to gain information about something in a secretive manner, or in other words to do reconnaissance. It is a military term and used when spying on an enemy, usually to aid in planning an attack.
It comes from an obsolete French word which mean to recognize. And because it is French it has an alternate spelling of reconnoitre, which is the preferred spelling outside the United States. The spelling variation extends to all its forms including: reconnoitered, reconnoitred, reconnoitering, and reconnoitring.