Kept private or not revealed

• Conducted with or marked by hidden aims or methods
• Hidden from general view or use
• Not openly made known
• Any of various organs that synthesize substances needed by the body and release it through ducts or directly into the bloodstream
• Not open or public
• Squirrel, at times
• Having an import not apparent to the senses nor obvious to the intelligence
• Designed to elude detection
• The next to highest level of official classification for documents

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