YIKES! Pool leak

I'm a new member of this forum, although I have consulted the forum for answers in the past.

I have a 10 year old gunite pool, approx. 20,000 gal. It has a spa/waterfall feature built as an integral part of the pool. In the last couple of weeks, I notice the water level dropping at least an inch a day. I'm in the humid Southeast, so we never see this kind of evaporation.

Needless to say I'm nervous, with visions of digging up the landscaping to get to pipes. My question is... where do I start? There is nothing visible (no cracks, holes, broken pipes, or soggy areas). Is there a methodology to investigate a pool leak? Should I call the original builder? A pool repair service? Who handles these things? Who should I trust?