If you want to see the stripper taking her clothes off 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.
Eliminate the first two targets on the 'Shaving Lenny - Streets of hope' level and then return to the door that is locked. Unlock the door to start a new level and then just walk straight until you reach a dog cage to see two dogs inside making out.
In the mission 'Hunter and hunted' look for a man waiting in a dead-end alley. 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.
When you are inside the 'Enter Industries' building in the 'Dexter Industries - Factory Compound' mission shoot the tip of the nuke hanging from the ceiling. This will enable you to view a hidden cutscene featuring Kane and Lynch.
Look for a side-room on the first floor in the 'Rosewood - Orphanage Halls' mission 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 start of the 'Dexter Industries - Old Mill' mission you will see 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.