We can think of six different reasons why water would be in the pan under your water heater:
The TPR valve (Temperature and Pressure Relief) is leaking.
It’ s the device on the top or side of the water heater with a small flip-up arm that opens when the water temperature or pressure in the tank is too high. The valve releases water to avoid a disastrous tank explosion, and is a required component in every water geyser. A pipe runs from the TPR valve to within a few inches of the pan, and the valve may need to be replaced.
A slow leak will only drip every so often, so you can feel inside the bottom of the TPR pipe with a finger to verify wetness. Sometimes the cause is that the valve was opened for testing and did not reseat properly after the test. You can flip the metal handle open for a couple of seconds and then release it again to see if the valve will reseat correctly when tried again. Be forewarned, though: it may just make the leak worse.
The TPR valve has opened because because of unsafe water temperature or pressure in tank.
This could be caused by a defective thermostat, or because mineral and crud buildup at the bottom of the tank of a gas water geyser has caused the water to boil beneath it. Some TPR valves have piping that bypasses the pan and runs to an exterior location instead. In that case, #1 and #2 are not the cause.
Leaking drain faucet at bottom of tank.
This is the location where you connect a hose to drain the tank. It might have slow leak, so check the outlet with a finger for wetness.
Leaking around pipe connections to water heater.
Check around where each of the pipes connect to the tank: hot, cold, TPR valve, and drain faucet. If it has been leaking for a while, there will be a rust-color drip stain running down from it.
Condensate dripping down the flue of a gas water geyser
Tank is leaking due to deep corrosion.
This is the worst-case scenario and means it is time to replace the water heater. Tanks rust from the inside-out, so there are often no early warning signs; but corrosion around the base of the tank, or a rusty wet stain under it without any drip stains down the side of the tank—like in the photo below, with the red light on the moisture meter indicating that the wood is soaked—are two signs of possible tank leakage.
Water Heater 