ART THIEVES? Multi-handicapped students at Santiago Middle School in Orange were delighted that their handmade ceramic plant holders were chosen for display at the Very Special Arts Festival at MainPlace/Santa Ana. Maybe someone else thought they were special too. The artwork has disappeared. . . . MainPlace officials say it may be just a mix-up instead of a theft. "If anyone comes across them, could they let us know?" says Cynthia Dobson, the class's instructional assistant. "What would anybody else do with them?"