The only inputs for the red BRAKE light on the Altima are the low fluid sensor in the brake fluid reservoir, or the E-brake.

Assuming the e-brake is disengaged, check the fluid level again. Even though it may be full, some cars need a little extra to keep the light off.

Keep in mind, brake fluid levels drop only due to a leak, or pad wear. In a perfect world, as all 4 pads wear, the brake fluid level will drop and activate the light at some point before the pads are totally worn out. It rarely happens though. To make a long story short, if your brake fluid level was low before you topped off...check your pads as they may be wearing the end of their life. Cheaper to replace before they start scoring the rotors.