The earliest level 5 hack

Probably not.

H/T, @Mana, via chat.

This post is a wiki post. Please feel free to edit it, but with first-hand information only.

First-hand testing has established that the Ghost rating is still achievable with:

• A camera becoming "suspicious",
• Turrets seeing you, firing at you and getting destroyed by you, as long as no living enemy seen/heard it,
• A guard getting "alarmed" by seeing/hearing an effect, like a very short (<1 sec) visual PC presence, a door opening, jumping or gunshot,
• Killing or knocking-out enemies with a takedown from behind, if nobody saw you,
• Insta-killing someone (probably with a headshot), as long as they didn't have a chance to see you & react, and nobody seen it,
• Insta-knocking out an enemy with a tranquilizer dart to the head (you'll see the Marksman reward), if nobody sees it - note that not every enemy (particularily those with heavy helmets) can be insta-knocked out.

However, you will not get the rating if:

• Enemy sees player for a bit longer (getting Hostile in the meantime), touches player or sees him carrying something,
• Any enemy sees a bot/turret destruction, ally death or an unconscious/dead body,
• Enemy sees a gunshot (only hearing it is still OK!).

Note that having a "Hostile" enemy doesn't automatically mean you lost "Ghost"; e.g. a lone enemy attacked by reprogrammed turret would get Hostile, but you still can get Ghost in that situation, if all the other conditions are met.

For more reference & test cases, see http://deusex.wikia.com/wiki/Ghost_%28bonus%29

Note that the Smooth Operator reward is an "easier" version of Ghost, which only applies to avoiding triggering installed alarm-systems, not detection or combat with guards themselves. (It cannot be rewarded for passing areas that lack such systems.) Smooth Operator is used for the Foxiest of the Hounds achievement.