Hmm, have you tried bleeding the brakes? Or do they feel spongy? Wonder if air is gradually returning to the reservoir and the fluid is replacing it.
The fact its drinking fluid would indicate its leaking though, and the point about having no marks on the drive indicates it could be leaking into a sealed or reasonably sealed area, not good!
Have you tried pressing the brake pedal and watching if the fluid goes down after 10 or so presses? If its leaking its probably more likely to do so when there's some pressure in the system.
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