Questions from First Time Pool owner

I have concern with water level, I hope this forum can help me. Thanks in advance.
I did try to use some big pail, fill it with water to compare the evaporation. I notice that the pool can go 1 inch lower, in just 2 days.
I read through the finding leaks section in this forum, but can't really get a lot. I remember earlier this year when the PB came over to change my pump, I share the same concern to him. Since water level stayed right beneath the skimmer level when we opened the pool, PB put some sort of color liquid only around the skimmer area to test if there's a leak around there. He didn't find anything.

So I guess I have 2 questions:
1. Does anyone knows what kind of liquid he used? So I can use to test for leak all around the wall (within the level where the skimmer) is?
2. Is there any other method to find wall leak such as this?

Thank you all
 
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