Can I suggest again that you swab the cylinders one morning after running the day before?

Your description of pressure in the cooling hoses lasting sometimes days after running it is exactly what I had. I agree it sounds like exhaust pressure entering the cooling system...what tends to happen, is that after the car is shut off the excess (higher) pressure in the otherwise sealed cooling system forces coolant back into cylinders.

I mentioned earlier in this thread that I had to re-do a head gasket on my 3.0 E53...I now believe that due to failures in both the elect aux fan in front of radiator AND the fan clutch behind the radiator, My E53 overheated when my kids were driving it and warped the head. It overheated later on me in traffic AFTER I discovered coolant in cylinder 6 and did the head gasket a second time. I replaced both the inop aux fan and the fan clutch and no problems since.

Oldskewel please swab your cylinders one morning.