'99 Sable GS ABS Light

Greetings!

I am the owner of a '99 Sable GS.

When I turn the car on in the morning, for about 5-10 minutes, the ABS light is illuminated. After the car has run, and presumably heated up, the light turns off.

What could be causing this? It does not seem like it's a wheel sensor issue as it doesn't come on during acceleration, or at random times and the car is mechanically very new (70k miles) for a '99 so I doubt there's any buildup. Also, as it seems to be heat related I'm at a bit of a loss. I thought the ABS system was mainly electrical (a box connected to sensors) and not directly related to the fluid in my brake lines/system itself.

Has anyone experienced this, or have any ideas?

Thanks,

-John

I should add - once the ABS light turns off in the morning it stays off for the rest of the day, even if I take a good 3-4 hour span between my next drive. I live in the Southern US, so the car may be retaining enough heat to keep the sensor off. However, as I said before, I'm at a loss because I did not think the ABS system was fluid based, so do not see how heat would "fix" it as there should not be any expansion in the system (as far as I know).