secretive

A secretive person keeps things hidden and private.

When people describe someone as being very private or mysterious, they might use this word. They could say, "My boss is quite secretive about his new project" to mean that she doesn't share details with anyone. This often happens in jobs where confidentiality is important, like law enforcement or finance. If you're trying to figure out what's going on but the person in charge isn't telling, they might seem secretive too.