I had a cavalier with a bad head gasket. No smoke, no oil in coolant, pressurised system to 15psi and held pressure. The head gasket was just starting to go bad. Only under the high pressure of combustion, would it leak. I saw air bubbles entering the over flow bottle at 2000 rpm.
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There is a simple symptom that one would see if there were a leak in the system. Aside from white smoke or the FML Fairy visiting you in your dreams to tell you exactly what she did - 99% of the time there is fluid where that particular type of fluid does not belong. Simple yeah? Now, try to find it. Could be water getting into the tranny lines on an A/T which would cause drop in pressure and room for air, could be, as was previously said, foam or white-ish sludge in the oil (this can also be from short trips where the car does not get to operating temp and stay there for very long, ie. "Been working fine running around town on short trips..."), it could be a small hairline crack in the block that goes from a cylinder to a water jacket.
Here is the 1% of the time example of when this is not happening. When I was a teenager I had an '87 SS Monte Carlo. It started to have temp issues. I could not see a leak. I drained all of the fluids and did not find a drop of cross-contamination anywhere. Eventually it would start to get hot and I could see steam coming from somewhere but could not find it. After new hoses, fan, radiator, eventually I pulled it into the garage and started to tear it down for head replacement. As I was taking the carb off I hit something with my ratchet on the back side of the intake. There was a T there that fed water into the intake on that SOB. I have never ever seen that set up before in my life, nor had my stepfather who has been monkeying with cars for a long long time. As I knew that it was not needed on any other vehicle I had ever seen in my life, I put a normal straight splice in the hose, tapped the hole in the intake and put a bolt in there with lock-tite on it. I never had another problem with that car again.
Sorry for the novel. I know this really doesn't give you a straight answer to the issue, BUT I wanted to point out that while there is a usual suspect in these cases, there is the '87 Monte Carlo water into the intake for some backward reason to consider. It could be a 5 cent fix is what I am getting at. Could be something small and insignificant, I would dig around in there before pulling the wallet out for anything major. Another pair of eyes on it may help too. Keep poking and you will find it eventually.