I just went out to my garage (I have 4 boxes of new front pads for my car and a brake wear gauge). My NEW unused pads are 10 mm thick, but I'm sure some new pads are 12mm else why would the gauge set include that number? The wear gauge is green for 8mm, 10mm & 12mm. It is yellow for 4, 5, & 6mm and red for 2 & 3 mm (replace).
The wear gauge set is only $12 on Amazon and takes all the guesswork out of "eyeballing" and the fact that not all pads have that wear slot in the middle
https://www.amazon.com/dp/B003QHSHM...=xh1lX&ref_=sbx_be_s_sparkle_mcd_asin_1_title
My two cents to you is that those pads are obviously fine and the stealership was being dishonest to your aunt, but that doesn't solve the issue of the BRAKE light coming on or the low fluid level. I think you know the answer to the fluid being low and it is not good. What I find most shocking about your story is that brake fluid level DOESN'T "naturally" go down at all, there is a leak somewhere; unless the brake fluid level sensor is going bad. I would THINK that a diligent dealership would look for/at all these possibilities, since repairs would be $$$$ in their pocket. My parting shot is I wouldn't take my kids little red wagon to a known dishonest dealer.