I would try to flip it a few more times. It could be some sediment got into seal and is leaking.

Dirt and sediment are prone to clogging up the spring and seat mechanism of the relief valve. It is acually recommended by the manufacturer to periodically test trip the relief valve. Trip it again for 10 seconds. If it still drips, drain the water tank below the level of the valve (not necessary to entirely drain) and replace it. Make sure you purchase a combination temperature and pressure relief valve that has the same rating as the old one.