To this day I still do not know why people come to internet forums and ask peoples advice. Most criminals get caught because they either brag about what they have done or ask about if they did the right thing. Now, I will add to this its obvious you are not a hardened criminal and probably never even stole anything, not even a grape at the grocery store but do you honestly think anyone here can tell you its ok to keep it?
If you want to know the hard facts, its true, it was marked as lost and fedex paid out to apple because once its shipped, its their right to reimburse the person who shipped it. Now of course it pends on if they have insurance on what was shipped but I would think Apple had some kind of setup with the shippers.
So, if you want a free conscience then call apple and tell them, end of story. If you feel the desire to keep it and use it or sell it or give it to your uncle bob, none of us here can tell you to do it or not to do it.
There is a time to stand up and make your own decisions in life, and well, this isnt something like "should I ask out betty joe," you are dealing with real money and real items.
Now will you go to jail if they find out, no, this is not theft in the traditional sense. To be a theft you would actually have to con a company out of actually delivering the product to you with an intention of fraud or using an exploit to buy it etc etc.
Process this all as you will but we are not yoda... To return or not, we cannot tell you this.