Subaru Brake Warning Light

I have a 1997 Subaru Outback with ~123K miles. Recently the dashboard brake warning light has come on. The brake fluid level is where it should be and the hand brake trigger button is functioning correctly. The brakes themselves seem to be working as they should. Sometimes it seems the warning light will come on when the ambient temperature drops below around 40 degrees.

Does anyone know what types of issues, besides the obvious ones I checked for, will make the brake warning light come on? Thanks.