Load the 'Dexter Industries - Factory Compound' mission and go inside the 'Enter Industries' building. If you then shoot the tip of the nuke hanging from the ceiling you will be able to view a hidden cutscene featuring Kane and Lynch.
On the first floor in the 'Rosewood - Orphanage Halls' mission look for a side-room which has a desk with a lit green lamp on it. Search near it to find a Mini Ninjas figure, Mini Ninjas is another stealth-action game made by the same developers as Hitman: Absolution.
There are two soldiers playing a video game on a TV and arguing at the start of the 'Dexter Industries - Old Mill' mission. Kill the soldiers and look at the game you will notice that it is very similar to Mortal Kombat.