Yeah, I checked from the top with a very strong light with the engine cover off both yesterday and again today and could see absolutely nothing neither at the bottom tray nor anywhere near the water pump or hoses. The coolant level also has not changed after one day and a a couple of short drives.
I cannot check from underneath where I am today. But it will be easy to check when I get home.
As for distilled water, yes. Unless it's an emergency, you should use distilled or "demineralized" water. You don't want deposits. Not the end of the world for small amounts, of course.
I could easily just have added distilled water to top it off, but I wanted to play it safe and also was worried I may need more than just a little on the trip back so I got the OEM coolant from the dealer. The coolant is concentrated (100%) and should be diluted to between 50% and 70%. I ended up adding about 1.5 cups of coolant and a cup of water to bring it to the top mark. Still no leaks I can see and level is holding. I have the better part of 2 gallons of mixed coolant for the trip back, but I hope not to need it.
To be clear, the temperature was always steady and I only get the warning once yesterday at start-up. I shut it off immediately and only started it again after topping it off. Obviously no warnings at all now.