The brake fluid was low but the problem persists even after the fluid was full. There is no visible leakage anywhere that I can see..
You could have a bad brake master cylinder, but you better get a mechanic to check it out first before you just replace it as a guess….its kinda a pricey guess.
If you push on the brake pedal (with engine on) while you are at a stop sign and the brake pedal slowly sinks to the floor…yes, I would suspect a failure inside the master cylinder and would advise replacing it.
There are ABS wheel sensors on the front, look for wires going to the wheels and you should see the small sensors on each of the wheels. If you have 4 wheel ABS you will have sensors on all 4 wheels.
Here is a video showing you how to test those ABS sensors
I would also pull the wheels off and check all the brake parts for brake fluid leaks or wear. Any brake fluid leak like in a front brake hose will cause fluid loss and the brake pedal to sink to the floor.
If the brake pads are totally worn out it will cause the brake pedal to travel farther to the floor and could turn on the lights as well since the brake pedal is lower than it should be.