This may or may not indicate a serious brake system problem.

The brake light comes on if there is an imbalance in the dual brake circuits using a rather simple tapered slide piston situated in the brake line distribution box (way below the master cylinder) and "floating" between the two brake circuits. The piston moves off center if there is a pressure imbalance and trips a pin switch which turns on the dash warning light. Once it trips it usually doesn't reset by itself.

This can often happen just after the brake system has been bled or had other work done. In that case it is typically a sign that air was (or still is) in one of the circuits or that one of the brake pads (pistons) was forcibly retracted too quickly. Otherwise you're looking at an operational failure which could be something like the following:

o Air in the brake lines. If the brakes have been "soft" (indicating air in the system) and you brake hard then the switch will trip. The system may not have been properly bled and purged of all entrapped air. Another way for this to happen is if a chamber in the master cylinder fluid reservoir runs dry of fluid (which is well below the MIN line). Based on what you say that latter condition wouldn't seem to be the case here.
o A leak in the system. Inspect for leaks. Places to look include the rubber brake hoses at the wheels, brake line fittings, the brake caliper piston seals (may need to remove pads and fully retract pistons to feel for fluid inside the dust covers), the master cylinder (including feeling inside the rubber boot of the master cylinder plunger connecting to the brake pedal) and around the brake line distribution block (the switch there has been known to leak when a slide piston seal goes -may not be externally visible so remove the switch to check for any sign of fluid in the switch outlet).
o If one of the rubber brake hoses has deteriorated or become damaged (either externally or internally) it can bulge or collapse during braking which will cause an imbalance. Have someone stomp on the brakes while you carefully feel each hose (ideally just after running the engine so you have brake boost).
o If the system hasn't been bled in ages or if old or open fluid was used (fluid will typically be dark in such cases) then it's possible for water to be adsorbed into the fluid. During prolonged heavy braking (as when going down a long hill or when towing a trailer) the water can suddenly boil, turn into vapour and create an imbalance, not to mention a dangerous braking situation. Residue in the bottom of the master cylinder reservoir is also a sign the system hasn't been properly maintained (I recently saw an awful example of this).

You need to determine the cause and correct the situation. At the very least a full brake bleed is likely in order. Obviously the brake system is fairly critical so if you're not mechanically inclined then head for dealer service or a good import repair shop. Franchised brake shops may be okay, but many will oversell you on repairs and parts. Until you figure it out, I would drive this car slowly and carefully. With the triangular dual circuit setup your car has, you always have something like 2/3rds braking power -you just don't want to be in a situation where you need that other 1/3rd.

If you can identify the problem and deal with it yourself you will probably still need to reset the switch at the distribution block. This is done by removing the switch, stepping on the brakes to center the tapered piston and re-installing the switch (yes, it's a bit awkward to work down there).
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day