I'm so sorry! It freaks me out every time one of our cats has gotten out. Even strictly indoors cats still have their instincts, though, and most likely Sebastian is just fine and will come home when he's hungry.
If you do think he's in the area, do you ever give him cat treats at all? If so, walk around rattling the bag -- cats are super-sensitive to sound and if he hears it, he'll come running. This is how we've always found our cats.
I also second the advice by others -- put up posters, offer a reward, get the neighborhood kids to watch, put an ad in the "lost and found" in the paper.
I hope he comes home soon so you can stop worrying. Take care.
-------------------- jen
"It's one of the most serious things that can possibly happen to one in a battle -- to get one's head cut off." -- LC