Sandy, it is also possible that the leak could be in or around your tub. The drain in the tub can move in funny ways when someone is standing or sitting in it, allowing a leak. Same with the tub itself or the caulking could be missing or damaged. Just running the shower would not simulate this. Also, water can follow the wood that's inside your walls in ways that aren't always predictable.
It can get to be very frustrating and expensive to find this kind of leak. Pro's have the same problem with this, perhaps, a little more insight as to where to look and the probabilities and possibilities.