check to make sure the overflow hose has no nicks in it and seals well.. other than that, you've got me stumped. (basically it is the coolant cooling off, creates a vacuum to draw the fluid back from the overflow, so a pinhole leak may cause it not to draw back in.. I remember one time got my jeep real hot and I could stand and watch it suck back from the overflow bottle, literally see the fluid level start to drop in 30 seconds)