You didn't mention the model of vehicle, but try wiggling the sensor wires prior to rolling the car. The other likely possibility is the brake light switch. As soon as you release the brakes and the car starts rolling, the ABS light will come on if the switch is faulty. This can also be temperature dependent. It is on my C280.

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'96 C280 (gone)
'97 C36
'05 C230k