Coolant leaking from back of the motor once operating temperature is reached. What can this be?

 

I'm not really a mechanical guy. Always had mechanics working on my car. But I sometime have good sense of logic.

When you don't loose coolant, but suddendly, when the car warmed up, you loose coolant, It must be due to something changing satatus when it eats up. Speciifcally, I would suspect some joint (how do you say joint in english ?)... said otherwise, some rubber parts that start to become less well fitting when heated up. Maybe the gaskets.

But it's not a very valuable opinion I give you. I must recognize that. Can't you identify for where is it leaking exactly (difficult to see in the area maybe) ?