If it's not leaking on the ground then it could be a small breech in
the head gasket. I don't know what else it could be.
Does anyone know if the catalytic converter  would mask the symptom
from the exhaust?

Then the test was not performed properly, or you have internal leaks.

Timing cover coolnat leaks will cause coolant to mix with your oil. That
should be easy to see on the dip stick.

Intake gasket leaks may cause oil and water to mix or may just leak coolant
into the intake system.

coolant? I'm trying to make a decision about

Have a technician pull the spark plugs and read them for signs of coolant
getting into the combustion chamber. An experienced tech should be able to
tell. If they all show signs of coolant then the intake gasket is leaking.
If only one or some of them show signs of coolant then the head gasket is
leaking and /or you have a cracked head. A pressure test with the spark
plugs removed or using a gas analyzer to sniff the radiator filler will also
verify a leaking head gasket (hopefully not a cracked head). Only
disassembly will tell for sure.

Another test would be a cylinder leak down test. That will tell you if
compression is getting into the cooling system and it will indicate which
cylinders are affected.

There are ways to diagnose your problem. You just need to find a qualified
technician. (sometimes easier said than done), but if you ask him about
performing the above mentioned tests and he seems to understand how to do
them, then at least you've got a chance.