If you have an air bubble in there you could have a max temp guage and the water be cool. Fill the coolant from the top hose.
The word "AIR" from air hose is on top of the fill point for the coolant.
This is the highest point in the system and if you read your manual this is where they want you to fill from to eliminate the air bubble problem.
The plug is plastic so becareful removing. I have to use a vis-grip to open mine. You may not have this problem though. Mine has buggered threads.
(unlike this post thread, which I have yet to bugger)
As far as your leak location. I am thinking maybe you have a leak near or under the overfill tank. If you have coolant on your oil filter that would tell me to look at the coolant hoses on the passenger side of the engine.
