How can a sprinkler valve be missing? It isn't as if it can be picked up and moved right. Well, this may seem like a strange problem, but it isn't that uncommon to be unaware of the location of a sprinkler valve.
If the sprinkler system installer didn't provide a diagram or if the system was installed by a previous homeowner the precise location of every sprinkler valve can easily be unknown. This is especially true of below ground sprinkler valves.
If you find yourself in this predicament take a look at some suggestions for how to find a sprinkler valve that can help make your search a little easier.