Designed to elude detection

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

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