Before Volvo 240's got ABS, they had a dual (redundant) triangular braking system whose warning light was based on a pressure differential between the two circuits. For example, the front calipers had two brake lines (one from each independent system, which was also to one of the two rear calipers).

But with ABS, as your '93 has, the cars no longer had that redundancy in two separate braking systems -- thus, each front caliper now only has one brake line.

So now, without the warning light being based on a difference in pressure between two independent systems (because there is only one), Volvo put the warning light's actuator (in the form of a level sensor using a float) in the brake fluid reservoir.

Your light is going on because it's marginally low in the reservoir. At rest, or at a steady movement, the level is high enough to keep the light off; but going around a turn, the level in the reservoir tilts enough to lower the float and cause the light to go on.

Not only refill the reservoir, but also (obviously) look for the leak!