To unlock the stripper at the window in Sniper Challenge you must first shoot three items. First item is located on the bench the second item is located next to the pool and the third item is located on top of the bear.
On the 'Shaving Lenny - Streets of hope' level eliminate the first two targets and then return to the door that is locked. When you unlock the door and the new level starts just walk straight until you reach the dog cage to see two dogs inside making out.
Look for a man waiting in a dead-end alley in the mission 'Hunter and hunted'. When you find him knock him unconcious and put his body in the dumpster. You will then find a chipmunk mascot costume on the ground that can be used as a disguise.
Go inside the 'Enter Industries' building in the 'Dexter Industries - Factory Compound' mission and shoot the tip of the nuke hanging from the ceiling. Doing this will enable you 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. Near it will be a Mini Ninjas figure, Mini Ninjas is another stealth-action game made by the same developers as Hitman: Absolution.
At the beginning of the 'Dexter Industries - Old Mill' mission there are two soldiers playing a video game on a TV and arguing. If you kill the soldiers and look at the game you will notice that it is very similar to Mortal Kombat.