Word or phrase like "underground' but hidden in the air not beneath the surface?

I am looking for a straight forward word or phrase to describe an occupation that is essentially underground/undercover work but I specifically want to stress that the work happens in the air/above the ground (sky, mountains, trees etc) not below the ground as underground implies. The specific meaning I am looking for is that it is hidden in terms of not physically visible or easy to get to, but also that there is a level of secrecy amongst the employees.

Looking at these two words in more detail and what I’d like captured from them:

• Underground: in concealment or secrecy; not openly:
• Undercover: working or done out of public sight; secret; engaged in spying or securing confidential information:

Examples of types of occupations that would fit the above:

• Stealth aircraft pilot
• Surveillance person mostly situated at a high vantage point
• Watch tower guard working at a top secret facility

Example sentence (using ‘airborne’ as a placeholder):

• When I was airborne I understood what I had to do, but now that
  I’ve gone underground, nothing makes sense.
• When I was working airborne, I enjoyed the work. Now I’m working underground and don’t know which way is up.