Brake master cylinder and booster pump

if the brake actuator were bad, there should be a code. That would be the real confirmation.

I think the battery you put in is not a good battery or it wasn't fully charged when you installed it. I would probably return it and get another one.

I don't think the actuator would run long enough to drain a good battery

 

There are simple tests for parasitic draws with a series ampmeter. Know how big of a draw you have first before spending money for parts. Parasitic draws are somewhat common and are often aftermarket gps, radios or obd2 devices.

Measure and report back. If your mechanic did not suggest testing go elsewhere.

 

It's easy to hear the brake pump running, so that's an easy check for whether it could even be the culprit.

In a good unit, it should run hardly at all when the brakes aren't being used, like, you'd have to leave the car turned on for an hour or so to hear it run again.

Many people keep driving with obviously leaky units with the pump running as often as every ten seconds or so, with nobody using the brake—which, despite being something like a hundred times worse than a good unit, often won't have set any trouble code yet.

If it kept on doing that, every ten seconds, all night while the car was off, I could easily see that draining the battery. The thing is, I don't think it does continue to run indefinitely with the car off. I think it may keep running like that during the first minute or two after turning the car off, and of course will begin again as soon as the driver's door is opened the next morning. But for the bulk of the time the car is off, I think the brake system does shut down.

So that's why I would be skeptical of the brake pump explaining the overnight battery drain.