I'm also facing this coolant leak issue at the moment in my 4.2 V8. It's probably leaking behind the motor, from the left side if looking from the front of the car. I can see crust around there. Car is going under maintenance soon so they can confirm this.

Edit: Cause founded. It's again this famous oil coolers 1" plastic pipe. Don't know what's going on behind the motor, maybe motor is just "swetting" a little bit.

 

My 2009 3.0 Tdi Q7 has the same problem. It leaks like 2 l at every 300 km and it drive me nuts. No drip, no white smoke on my exaust, no presure in my coolant reservoir even when the engine is hot. I've changed the EGR radiator with no results. Can't see the leaking spot. I can't see any coolant stains on my engine either and everything is clean and dry. Something is happen when the engine is hot and running. I've heard that is a stopleaking solution but not that we use for radiator ... one which cover the tiny pores on the engine level. Guys, you have any idea about how I can fix the problem? An engine change is out of question... Thanks,

 

My leak seems to be behind the motor as I've read others having the same problem. Can anyone tell me if you can get to it from below or what has to be done? How much it cost for a mechanic to fix?