Hiro from Mini Ninjas
While it is not really a cameo or anything, during the mission at the Rosewood Orphanage keep an eye peeled for little Hiro dolls situated throughout the rooms.
Kane & Lynch Guest Appearances
The characters from the Kane & Lynch action-adventure shooter series put in an appearance in Absolution, apparently on a shopping trip for kit for their next heist! Kane is encountered in the Biker Bar in Hope, and Lynch is found at the Gun Range.
Loch Ness Monster
While infiltrating the Burnwood Mansion you end up on the balcony overlooking the sitting area that has powered blinds on the windows. If you hit the switch to raise them and then quickly shoot the pair of seashell statues, when you look out the window at the water on the left you will see the Lock Ness Monster come swimming by!
In the inside of the Dexter Industries Factory there is a Nuke Bomb suspended from the ceiling that, if you shoot he tip of it, triggers a CS in which you get to see the puppy go off! Of course that ends the mission as a fail, but still, funny, no?
Easter Eggs are special events, items, in-jokes, homages, and other elements that are inserted into a video game by the developers as an added bonus for players. They are often hunted for by players who take particular delight in being the first to discover them. Hitman: Absolution contains a few (probably more than we have learned about) Easter Eggs.