after reading the article posted by daniel, and seeing the age this started at and the age the child is now, i am wondering if the initial "stealing" of things wasn't responded to properly by the parents. if it was ignored and not talked about, it may be that it's become a habit. the article states that after age 6 a child has a sense of what's his and what isn't, so maybe just a little talk as suggested in the article is in order.
HI, my son is now 20 yrs old, an only child. I dealt with this when he started 2nd grade and another child's parent called me to say my son had borrowed and not returned a book... my son denied it and until I found the book mixed in his toy box, I thought the other child had a lying problem. This was a symptom of another issue in our case but hope that in yours it's a phase that can be worked through. If you feel (and you would) that there is a bigger issue then that's different but if it's just this squirrel type behavior I think you will be okay. good luck