It usually means the brake fluid is low. Check the reservoir. Could be a leak in the system or worn brake pads.

If both those check out, it could be a faulty sensor.

 

Brake fluid was good. I'll check pads and leaks tomorrow. It's weird because the car wasn't used for a whole day and suddenly got that warning. Thank for the reply

 

Maybe there's dust on the calipers. What's the year and trim of your car?

 

I think it's going to turn out to be a wheel speed sensor. Not that unusual.

 

That would be an ABS light, not a brake light (or would be both) - not just a red brake warning light.

 

Could it just possibly be a dirty sensor in the master cylinder?

 

Or more likely a loose/dirty connection to that sensor