I had a blue tang in my 150 gallon tank that died. I'd had him in my tank for over two years. He had wedged himself into some live rock and I think he may of gotten stuck. Here is the problem with Tangs: if they don't have enough space they can get stressed out. They are not all that hardy as they can get ich, lateral line disease, velvet...you name it. They generally like more turbulent water and their diet is crucial to living a long time. If they arent getting the right foods then they will not make it. These are one of my favorite fish. I can tell you that while you could have a juvenile in a 40 gallon tank (they are really small and cute!) for a short time, you would need to prepare to upgrade to a much larger tank as these guys like to swim and they can get rather large. I'm no expert by any means but I've had quite a few tangs (yellow, powder blue, brown, regal) and when it comes down to it, if you have had any sick fish in the tank...prepare for them to get it. That's just my experience though and I know that everyone has different opinions and experiences with fish. It's a hard learning process when you lose fish. Personally, I would not replace him with another tang. In addition, if you have two fish missing and possibly dead, there's something not right.
What all do you have in the tank right now?