Although coolant loss can be caused by a faulty head gasket, even without the tell telltale overheating and displacement of coolant, my experience has been failure of the the lower intake manifold seal. This can easily result in intake vacuum sucking coolant through the seal since there is ~45 psi difference across the elastomeric gasket. The seal often fails due to the thermally induced movement of the aluminium heads and intake manifold on the steel engine block. You can usually see it with the manifold removed because there will be corrosion patterns on the head and/or manifold where the seals make contact.
Unfortunately I didn't have much success with just a simple remove/replace of the gasket, I believe due to the corrosion at the sealing points, which prevented a good seal.
Check for bubbles in the coolant, or a rapid increase in coolant pressure on the cold engine before heating can be held responsible for the system pressure. Those are signs it is the head gasket.
This webpage may be helpful to determining if it is a intake manifold leak or a head gasket issue.
https://www.stangnet.com/mustang-for...in-oil.632038/
Also, check the plugs for signs of coolant (exceptionally clean, for example).
To repair, I found that I had to improve the quality of the surface to be sealed. I read that one of the identified causes of leaky intake manifold gaskets in V-6 engines is due to the thermal expansion induced movement of the gasket across the head face. When the machined face is too rough, the gasket material is damaged and leaking begins, getting worse over time as the gasket is further degraded.
I improved the surface finish by filling the corrosion pits with JB weld, then polishing the mating faces of the intake manifold and cylinder heads with ~1000 grit abrasive paper. I did this while the engine was disassembled for a rebuild, but I believe it could be done with the heads mounted to the block, but you'll need to be careful to control the debris from the polishing process.
I did this about 70,000 miles (5 years) ago, and I'm now starting to see coolant loss again, but this has worked better than replacing the gasket alone.