Just took the Runner out for a 3 hour trip.
Checked all fluids before I left and when I got there I noticed there was a hissing sound and the whole underside of the car was wet with coolant.
Also the reservoir was pretty much almost dry. Took about 3-4 quarts to get the reservoir topped off again.
Both waterpump and thermostat are less than 4 years old and the temperature gauge never rose higher than middle ground. Also there's no smoke or smelly exhaust fumes and the reservoir tank looks clean, oil looks nice and black still. Filled up coolant and there are no leaks, still holding after cool down.
Any idea what happened? I still have to drive 3 hours back home!