Car has started to use coolant

Hello,

It is possible to have a minor leak that only comes out when the engine is running and that could be the reason why you do not see anything on the ground.
Also, if you are just using water, it will evaporate fast, and you will not see any excess around the engine or radiator as the heat will evaporate it with not leaving any residue.

It will be a good idea to do a pressure test of the cooling system.
The pressure test basically puts pressure on the cooling system, similar to when the engine is running at operating temperature.
If there is a leak it should be easier to find.

Here is a helpful video of how to do a pressure test for the cooling system:

https://youtu.be/W8kra65m15c

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

Please let me know what you find or if there are any questions.
Thank you.