2006 Audi A3 Coolant Leak only on Long Trips

Hi,
I'm having major coolant leaks on long trips (4+ hours). I drive boston<->DC frequently in the summertime, and if I fill coolant to max when departing, after 4-6 hours it will be low enough that the "Low Coolant" alarm flashes and I have to stop and refill.

1. I use 50/50 water and the suggested Audi's pink coolant, got it from autozone

2. This does NOT happen on trips shorter than 4 hours. All winter long, I only filled the coolant once, while putting on ~4000 miles. It doesn't leak in my driveway.

3. The engine never overheats. The needle consistently stays half way between C and H.

4. When I open the radiator cap, barely any pressure is released. It sounds more like a cat or dog passing gas than the loud hiss that I'm accustomed to with other cars.

I've been looking / touching looking for weak or leaky connections but haven't been able to find anything yet. Anyone who works on Audis will know how congested it is under the hood.

What should I be looking for? Will a mechanic have some fancy tests he can run on this guy? I love my car and want to keep it well maintained!

Thanks,
Rajat Banerjee