What are the two most probable causes of low fuild levels?

One of the most general cause of low fluid is that the liquid leaks from a point in the system .

To find this sometimes the car repairman will pressurize the system to be able to reproduce where the system leaks .

This way they can see better where is the leak and fix it .

The second possibility is that the liquid gets warmed up causing evaporation and a low level reading over time .