I lost brake fluid but it didn't leak out

Hello,

If you are losing brake fluid, then it is due to a leak.
One of the most common leak area is the rear wheel cylinder. Those wheel cylinders have a tendency to leak out of the seal, and that will have brake fluid leaking inside the brake drum. Which can be difficult to see without removing the brake drum.

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

Keep in mind that replacing the wheel cylinders will require the brakes to be bled to purge out any trapped air.

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.