I have a 2006 Sep T5, and the coolant tank never stays full. Every time I add coolant to it it slowly drains out within a few days. There's never any liquid on the ground either. Also, my car won't heat up until I start driving, and the temp gage on the dash will never go above 45 degrees. I know these are all related, but every mechanic I bring my car to can never figure out what it is

 

Common causes for that is heat cracks on top of the coolant bottle or head gasket.

 

Does your oil look good (i.e. no signs of contamination)? How about the coolant? Do you get puffs of white smoke coming out your exhaust in the morning when you start the car? Those could be signs of a HG leak but don't go there until you've ruled out all other simpler things (ex: leaky reservoir).

I would not rule out a simple leaky rubber hose somewhere either...if the coolant is dripping onto something hot, then it might evaporate on the spot not leaving a trace anywhere for you to troubleshoot. Have a good look at both the upper and lower radiator hoses in particular to make sure they aren't cracked. Squeeze them and bend them a bit to see if they reveal cracks or feel very weak.

I would also try periodically stopping during trips to see if you can tell when the fluid level starts to go down - you may get lucky and hear some hissing or get a hint of a sweet smell somewhere in the engine bay as the liquid drips.

I suppose you've already changed your thermostat, right? Also, have you got the reservoir cap on nice and tight? I left mine undone one night and forgot (long story) and within 40 miles, most of the coolant was gone and I only got wind of it when the engine bay steamed up after a long highway run. Luckily I was right in front of the autopart store at the time!

 

Did any of the mechanics do a block check where they test for a headgasket leak at the coolant expansion tank ??
Usually if it's leaking and you can't find it then it's going into the engine (or oil).

Check out a youtube video: https://www.google.com/search?q=tdo4...e+block+tester

 

Check under the floor mats in the back seat footwell. Mine has a slow leak in the heater core and it's running under the carpet. You only get a hint of smell every once in a while.