"In the whole army, none should be closer to the commander than his spies, none more highly rewarded, none more confidentially treated" - Sun Tzu's Art of War
In no Shinobi are the hidden shinobi that would have been hired by lords. These could already be in position within enemy ranks. According to Chikamatsu Shigenori they could be hidden among a castle's own troops. They could also be agents still living in their own province being paid as full retainers. All arrangements and transactions done were secretive.
The Bansenshukai shinobi manual warns shinobi not to attempt to gain fame for their skills, as the more known an agent becomes due to his expert skills, the greater the chance of him being detected by the enemy. Detection was a real issue as political alliances changed throughout the Sengoku period.
Later in the Edo period, Chikamatsu wrote that his Koka master, Kimura, said that some lords did not fully understand how to use shinobi and how to run shinobi teams.