Also have a look at the welsh/freeze plugs on the near side of the block, they can rust through and start to dribble coolant, then when the level gets low enough for the block to really heat up they fail and spray coolant on the exhaust or under the car in a fine mist, leaving you on the side of the road wondering where all your coolant went.
Although there a three of them, it's usually the centre one that rots away first due to proximity of hot exhaust pipe and road salt.
Buggrit, another option is a cracked block if you refilled before letting the motor cool properly, I really hope its not that though.