Well, I would have to say that they will usually find stuff to do. I might have to say it's boredom but from some of the things I have seen, it's to let me know they didn't appreciate something. Some people would disagree.
I will give you some instances here at my house. My cats will sleep in the dishrack. Last time I left for a whole day, I came home to find it along with the dishes on the floor. My cats know not to get into the cubbord and NEVER do when I am home. When I have been gone for more than normal, I will come home to find they went in there and pulled the trash bags out. My husband found this to be very true because last summer, I went home for 3 weeks. He would call and tell me that they were getting into the pantry, on the counters with food there and such. I beleive it was because they didnt know where I was. As soon as I got home the hehavior stopped. I tend to lock things up and what I call kitty proof the house before I go anywhere just if it's a little longer than usual. I hope this helps, if not give me some specifics.