If your diligent water heater maintenance includes draining the water heater you may be rewarded with a leaking drain valve. This can happen when the valve is old and hasn't been used in some time. It can also happen with a new drain valve if it is a cheap plastic valve that often comes standard with a water heater.
In either case there are two possible ways to deal with this problem. One way is to cap the drain valve to contain the leak. The best option, however, is to replace the leaking drain valve. It is not a difficult thing to do, it is an inexpensive repair, and it will make it easier to drain the water heater in the future with confidence.