I had the Brake warning light appear while driving on Sunday night, 5 days after collecting my new Model 3. The message told me that the brake fluid level may be low and that I should find a safe place to pull over and contact Tesla. I should have taken a photo for the exact wording, but it did sound a potentially serious issue.
I used the app to report the issue and schedule a service. I also phoned the Service Centre on the Monday and spoke to someone there, who seemed familiar with the issue and fairly relaxed about it, although they did say the message was unlikely to go away by itself.
I didn't use the car after that, but took it into the Service Centre yesterday (Thursday). In my case, the people at the Service Centre did say the brake fluid was slightly low, so it wasn't likely to be a false alarm as such. They said the issue is likely to be that there was a small air bubble somewhere in the system. It sometimes happens if they fill it up too quickly in the factory, and it can take a while for the bubbles to work their way out through use. The same can happen with the battery coolant fluid sometimes as well apparently. When the bubbles have worked their way out that can trigger the warning about the fluid level. Topping up is usually the end of it, which they did. They don't think the warning will happen again, but if it does they said they would take another look to see if there is some other issue.
By the sounds of it, there are others with this issue too, so hopefully this information is of some assistance to those.
BTW, it was my first trip to a dedicated Service Centre. Seemed reasonably well appointed, with nice coffee, some children's play things, and doors to the service bay that open automatically when you approach in your Tesla. The staff seemed knowledgeable and helpful as well.
Got an approx 800 mile road trip starting on Sunday so hopefully all well for that now.