I would definately ban the kids from going in his room. These games aren't cheap! I was kind of in the same position last year. My 7 year took his gameboy on the bus and let his friend "borrow" it for the weekend. I didn't know he snuck it on the bus let alone let his friend borrow it. I didn't know this kid at all and my DS couldn't remember his last name! I didn't find out it was missing until later in the next week. I called the principal of the school - since it happened on the bus home and luckily the principal stepped in and called both children to the office. Discussed what happened and wanted the friend to bring it back to school to return to my DS. Yes, we did get it back, but it looked like it had been buried in dirt and scratches all over it. It still worked though. I scratched his name and our number on the outside of it with a screwdriver.
Hopefully, the parents of the other kids your child plays with will find it and get the little one to return it to its owner. But until then, I would again....ban his friends from playing in the bedroom Good luck.