Leaking coolant

A leaking cylinder head gasket is the best suspect when no obvious leaks can be found. Your mechanic can perform a chemical test at the radiator to check for that. You can also add a small bottle of dark purple dye to the coolant, then check a day later with a black light. The dye will show up as a bright yellow stain. If the head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the dye for the fluid being tested, and those that rent or borrow tools should have a black light.

The reason everything is dry is probably because it is only leaking when it is under pressure, which is basically whenever it is running. You turn the car off, it stops leaking, and then any coolant dries off over night.

They make a specialized air pump that connects to your radiator. You pump it up to 15 psi and then start looking/listening for a leak. Identify the offending part and replace/repair it. It really is that simple. The pressure tester can be borrowed from the local auto parts shop with a deposit. Here is a more specific how-to guide on the subject that we wrote: https://www.2carpros.com/articles/radiator-pressure-test

And ALLData's information on the subject is attached in the images below. Read both before doing anything. Let us know what you find and we will go from there.