You are a bit confused about shut off valves. The one HJ refers to the one you turn the water for the tub and shower on/off with. There normally are two other valves that are not as visible or used very often. These are small valves in the hot and cold lines prior to their meeting at the main on/off valve. These valves allow the water to the tub/shower to be turned off so the other big valve can be worked on without turning off the entire house. Normally, all sinks, toilets, and showers have these valves. Look under your toilets and sinks and you will almost certainly see them. As HJ and Terry point out, there are two possibilities for you problem. The big valve could need new internal parts or the water you are calling a leak is just normal water draining from the shower pipe. If it continues after a couple of hours, it just the normal draining. If it continues, then you need the big valve serviced. It is easy to be confused when the term, "valve" can be used in different contexts.