Is the fluid level changing at all? I presume you've checked it but I said I'd put it out there anyway. It sounds like it's getting air into the system somehow although the fact that it's not been driven half puts that theory out the window.

Definitely a strange one. I'd also go about checking all the brake hose and calipers for leaks, could omit be possible that a brake hose was caught off the ground somewhere? You could isolate the calipers by putting a vise grips on each brake line and leave the car for a while to see if the softness returns.

Is there any leaks around the abs pump?