Coolant leaks at the left rear of the engine towards the firewall?

Hello,

You could have a gasket leak. As it leaks when the engine is cold but as it warms up all the components expands and possibly seals the leak.

You should do a pressure test when the engine is cold. This will create pressure similar to when the engine is running, it will make it easier for you to detect the leak.
You should be able to use the tester from your local auto parts store.

Here is a helpful video of how to do the pressure check:

https://youtu.be/W8kra65m15c

https://www.2carpros.com/articles/radiator-pressure-test

Please let me know of any questions.

Thank you.