PS. I have no idea how a pool works...

I had a lot of water loss during the summer. It slowed down or completely stopped until about a month or so ago, meaning it is most likely evaporation.
Here's a good test:
Bucket Test
Get a 5 gallon bucket. Place it on a step and fill it with pool water so that the water level inside the bucket is the same as the water level outside the bucket. (Don't use the hose, the chemistry will be different affecting evaporation levels)
Using a marker, mark the inside and outside water lines. Leave it for 2-3 hours. If the inside and outside levels are different, mark the new water levels and not the elapsed time. Leave for another 2-3 hours and mark again. You should see no real significant difference between both inside and outside levels. They should drop the same-ish or hold steady the same-ish. If there's an inch or more of difference at the end of 6 hours, you probably have a leak.

How old is the pool and pool equipment? Is there a water feature or jacuzzi? Is the pool screened or open?
As for your father changing the settings, there's no setting combination that would cause water loss.

As for not knowing how a pool works, I can explain. The pump PULLS water out of the pool through the skimmer (which catches floating debris before it becomes waterlogged and sinks), main drain (on the floor of the pool), and vacuum lines (which suck up sunken debris). This water is first pulled a basket to catch the bigger debris, then passes through the pump and is now PUSHED (since it's on the other side of the impeller in the pump) into the filter housing. The water is forced through the filter and pushed back into the pool through the pool returns ('jets') or combination of pool returns and waterfall if you have one. The 'jets' push the water around the pool in a lazy circle so that debris is pushed past the skimmer, where it's sucked back in... the vacuum keeps picking up more debris... and the process begins again.

~Bridget