Here are the 8 hidden objectives.

Survival Expert - Mission 1 / Survive Gauge's onslaught after the download completes.
Master Thief - Mission 2 / Capture a Rogue Engineer.
Great Escapist - Mission 2 / Complete the operation without a unit being captured by the enemy.
Cache and Carry - Mission 3 / Collect all of the Technology Caches.
Sultan of Soul - Mission 4 / Build at least four Soul Ripper Experimentals.
Master of the Deep - Mission 5 / Build a Kraken Experimental Submarine.
Master of Pawns - Mission 6 / Win the operation without the aid of any Experimentals.
Research Savant - Mission 6 / Complete all available research.