Re: 2001 Grand Cherokee 4.0 losing coolant
? Got an oil cooler?
You might be able to spot half dried coolant on parts under the radiator, if you see any, you'll probably need a new [i recommend and aftermarket unit] radiator.
The plastic tanks and the plastic hose connections [and the cooler connection] can develop very small cracks that only lose coolant when the engine is hot and the pressure is up, so you wouldn't see them when it is cool, and as noted it tends to evaporate quickly. I had one that not even a traditional pressure test would detect.
The coolant recovery tank is another suspect...
If all else fails, invest in a small bottle of dye and matching UV light. Thoroughly clean the engine compartment including under the radiator so you can get nice clean results.