Q: Coolant reservoir empties in one trip

If the radiator cap has failed (i.e. it doesn’t hold the specified pressure built up in the cooling system when the car is running), the cap will prematurely open while the car is running and coolant will flow into the reservoir and possibly overflow the reservoir (thus "losing" coolant).

Then as the engine cools after you turn it off, all available coolant is drawn back into the cooling system to make up for the overflow, thus lowering the level in the reservoir, or perhaps even emptying the reservoir if the cap failure continues over time.

That latter scenario is a possibility, but radiator caps can be tested if you have the right equipment.

If it the cap has already tested, and the cap tested good and there are no cracks (leaks) on the plastic coolant reservoir or loose connections, that confirms that you have an external or internal cooling leak (no mystery - there is a leak).

A standard test for exhaust in the coolant should be performed to see if perhaps the head gasket is leaking. Cooling systems on cars are not like working on the space shuttle...that is your cooling system is very limited in extent and there are very few "devices" involved (thermostat, radiator, some sensors, a cap, and so forth.) I’m really not sure why the shop you took it to was unable to find the leak.

At any rate, the thing to do is have an appropriate diagnostic performed. A professional from YourMechanic can come to your car’s location to pinpoint the coolant leak and have this problem repaired correctly.