Possible Clues:

• Espionage figures
• Characters in Lara Prescott's novel 'The Secrets We Kept'
• Novel by Michael Frayn
• Seekers of intelligence
• Eavesdropping pros
• Notices Christmas fare if served with mint, did you say?
• Does some undercover work
• Moles, e.g
• Dead drop users
• Covert operatives