Won't be brake fluid, not on both sides of the car, almost defo your heater matrix, around £35 for a new one, about 4-5 hours to sort out, nice and easy job on your car, just get a haynes manual, if not ofa with cars would be a good one to learn on job wise, as can't really do any harm.
I know you say the levels not dropping but you'd be amazed at how little it takes, you may also notice a antifreezy smell when you turn the heater fan on as well coming from the vents, face one especially.
tabetha