Starcraft 2 Easter Egg - Hidden Tauren Marine

On the campaign mission named "Zero Hour" push your way through the Zerg's bases to the north of the map.
Scout around and you'll notice there's a neutral unit standing in the central-north part of the map.
Tool-tip help will reveal it is a LVL 80 Tauren Space Marine!
If you click or approach it with your units (I'm not really sure which action triggers it), it wil go into a small building nearby. Proceed to click on the building thrice and a launch countdown message will pop from it each time. Once it reaches zero, the building will fly away and the message "Secret Found" will pop instead of the countdown.
Tauren Space Marines were an April's Fools joke from Blizzard.