The important thing in the pan drain is that it flows somewhere where you would see that a leak has developed. Such as a visible floor drain or a spot outdoors that you walk by. An un-noticed leak has big consequences.
Is it just my aging eyes or does does anyone else see a smoke shadow below the gas valve possibly indicating flame roll-out. Also, the dark spot on the wall to the right of the heater; is that old water damage or charred?