Losing Coolant

[eje]

2000 Boxster

I was out of town for a few months, went to start the car and the low coolant light was on. I refilled the coolant reservoir, drove for just a few miles. Went to start the next day and low coolant light was flashing.
I refilled the tank, opened the engine compartment and access to the coolant tank. I ran the car to normal operating temp & until the fan came on. Shortly after the fan came on the low coolant light also came on and the reservoir was empty. It’s losing about 1 gal in 20 min.
There was a slight gurgling & hissing sound from the area of the coolant tank, but no leaks, no moisture either under the car or in the engine compartment. There was no moisture on interior or trunk carpets.
Once the engine is shut off and cools or when the cap is removed some coolant returns.
The car runs well, no smoke except occasionally on start-up.
The coolant tank was replaced by the prior owner a few years ago.

Where is the coolant going?

On my way to go find a coolant system pressure tester--