I'm sorry to hear that. Was your RES in the open like in the front yard (or something?) Or was it in the backyard? I don't know who would go into your backyard to steal a RES because I got my RES from chinatown NYC when I vistied and I can say they are very cheap. The thing is did you have a cover over the turtles? If you didn't theres a good possibility it ran away.
Here's a story of my friend who lost an adult RES. He had 2 RES and he was cleaning his tank. He figured he would keep it in a cooler since he didn't have a rubbermaid. The water was about half way and there was about 8 inches from the top to climb out. But still the turtles climbed out! Both! I helped my friend look for it and we found one hiding behind some bushes. We didn't find the other and not sure what happend to him.
So basically turltes can climb stuff that we can't imagine them doing if they wanted. Then there's the possibility of a bird or cat or other animal getting to your RES. If you don't have a cover over it that is a good possibility.
Sorry to hear it and I really think the only thing you can do is go and get another RES and hope nothing like this happens again.