My 1993 Honda Civic requires topping up of the brake fluid every 3 months due to the light coming on.

There are no leaks in the master cylinder and no leaks at the brake pads (I checked all 4 wheels). There is also no dripping of brake fluid in my driveway.

This has been happening for 2 years now. Any clues to where this fluid is going?

You probably know this in the back of your mind:
You've got to have a leak somewhere.
Since you're only adding it once every 3 months, it's no doubt very small.
Look close at every connection if you're super interested in locating it... but if it were me I'd rather check it every couple weeks and add a bit every three months than crawl around looking for where to spend some repair dollars.

Just my thought on it :)

You have a pinhole leak in one of your caliper boots. And that little over 3 months. Wow, you are so lucky. But yeah, that is where I would start. That or the drums on the back have a pin hole in them. You will have to pull the drum to check. But an easy way to check would be to get under it and look at the backing plate, you should see a small dark spot on the back. If so, you might consider getting a new caliper drum for the rears.

Let's see;
1 bottle of brake fluid (will fill up my reservoir 4 X) per year = $5.50.
Parts and service to replace unknown caliper and what ever else may be causing the loss of fluid = $1500.00.

I think I'll put up with it!

In automotive terms it is referred to as a seep, not a leak!

Received via PM:
I found it and fixed it. The bolts on the brake fluid reservoir were loose.

I noticed the black paint on the vacuum assist drum for the brakes was pealing off and saw a drip from the reservoir.

Case closed!!

Now that you solved the leak, this might be a good time to change all of the brake fluid.

Well, it's the Fall. I'll put that on the Spring maintenance schedule when I can do it in the warm sun.

Such a leak allows air and moisture into the system. Changing the fluid would improve the dry boiling point, wet boiling point, corrosion protection, brake responsiveness, and overall braking safety, among other things. Brake fluid is the most important component of the brake system.

I know. You can't stop without it! But mine is going to last another 6 months!