Losing brake fluid, but there is no sign of a leak anywhere

Hello, I am Danny.

Hmm, Since brakes are a sealed system the fluid has to be going somewhere. If you are not noticing on the ground maybe the master cylinder is leaking into the power brake booster. You might want to add more fluid a monitor if it is leaking down still. Definitely need to be looked at because no brakes would be dangerous. I am attaching a few links to explain symptoms and how to change a master cylinder. Hope this helps and feel free to answer any other questions if needed. Thanks for using 2CarPros.
Danny-

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder