Visit any shelter in your area, especially if you are near the boundary between two shelters. Do not call with your cat's descriptions, as you are leaving your cat's fate up to a clerk that may not know a Siamese from a brown tabby.
Cats do tend to travel.
Also post these at veterinarian offices in your area, shelters, and grooming facilities. Newspapers generally charge for lost animal ads, but many people read the ads. They may find that another neighbor thought the cat was a 'stray,' or someone may see the cat slinking under a building. Call to make sure your cat has not died and been picked up.