[Akmnhg5A]

If you had any recent brake work, that would be the first suspicion.

If not, and fluid a little low is normal as the brake pads wear. If you have to top it off again soon, you have a leak.

It could also be a brake booster vacuum leak. That's the big round can looking thing on the engine bay side where the brake master cylinder goes in the middle. There is a vacuum hose that connects to it.

I could be an internally leaking brake booster.

It could be a brake master cylinder with cup seals that are shot.