Can't answer with this limited amount of information. What was the procedure? What kind of mouth splint? Was the splint in the manufacturers packaging before it was soaked, or was it something that came out of the patient's mouth before the procedure and you were just putting it back? Was it being used for the purpose it was manufactured for?
The mouth isn't a sterine environment in the first place, so it's possible that this was OK depending on the procedure. But, like I said, there really isn't enough information to tell definitively.