"Normal" evaporation is different for every single pond. There are charts that will tell you the "pan evaporation rate" for your area, but you have to factor in so many things that are unique to your pond. I'd give it a few weeks and see if you consistently lose that much water daily. If it varies from one day to the next, then it's for sure not a leak. If it's consistent, you'll maybe want to investigate further, but you've already covered the basics - look for wet spots, check the plumbing, look for low edges... But honestly - it's rarely a leak. Waterfalls, wind, heat, humidity, plants, rocks... all kinds of things can affect the water level in a pond. You're right - common newbie freak out!