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There is no chance whatsoever that a one-time use will show up 20 days later. You can quit worrying about this. It is only regular users who have to worry about THC being in their urine days to weeks later.
Military testing is exactly the same with exactly the same cut-off limits as non-military testing. To test positive based just on exposure to passive smoke would require you to be in an enclosed room with several smokers 24/7 for the week prior to testing. Light exposure to second-hand smoke is not enough to make you test positive.
You have nothing whatsoever to worry about from your own one-time use of marijuana.