Conducted with or marked by hidden aims or methods

• Squirrel, at times
• Indulging only covertly
• Not openly made known
• The next to highest level of official classification for documents
• Communicated covertly
• Having an import not apparent to the senses nor obvious to the intelligence
• Hidden from general view or use
• Beyond ordinary understanding

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of secretor English)