Slick-fingered Bay Area thieves are stealing restaurants' used kitchen grease, a product that six years ago was so worthless that some restaurant owners would illegally dump it down the drain instead of paying for proper disposal. The leftover cooking oil, yellow grease, is the easiest material to turn into biodiesel. And as the alternative fuel's popularity has increased, so has its selling price, making it a target for thieves willing to go through the trouble of pumping or siphoning the grease out of storage barrels. "It's liquid gold," said Daniel Rugg, director of engineering at the Four Seasons hotel in...